Whoo, boy! This one I'll be at a while. I am NOT a luthier, let alone one who has experimented with lots of variations. I've read some, and I'm going to throw out some ideas and thoughts I have, in the hopes that they'll help. I'm going to reply to your questions out of order, with the simplest/quickest responses first.

Nick said

2) where is the best place to find *decent* but cheap aged wood and tools for this?  What tools will I need? (I like this list:https://www.violins.ca/tools/r....._list.html).  My goal is to make something of playable, professional quality, but not world class.  I'd like to spend less than $1000, preferably around $500 for this.

Pretty much everyone I've heard talk about tools references stewmac.com. Don't skimp on the tools, but you shouldn't need to - they don't don't run all that much, and they'll last a long time.

My teacher mentioned getting "white" violins (ie, unfinished ones). When they haven't been shaped at all, they're not terribly expensive ($40-50), and I don't imagine violas would be tremendously more, probably in the $60-70 range, possibly up as high as $120 if you wanted a 21 inch one.  Shaped ones, which only need to be stained and varnished, run a good bit more. You'd have to hand build a fingerboard, and possibly a neck.  I tried searching for them but had limited success. I'll ask him for websites next time I see him, but that won't be for two weeks. ("White violins" got me a bunch of violins painted white - "unfinished" got better results, but very few of them were the bare bones sort.)

I'll have more to say about the subject if the wood later, though. I don't think you're going to want to start with concert quality stuff.

3) thoughts on best ways to incorporate electronic component into it? I'm sold on the piezoelectric sensor in the bridge, but where to put a line out and where to put volume control panel and 9 volt battery (I would like to have those on this, unless someone says that's a bad idea)

There are solid blocks above and below the waist of the violin/viola, on both sides. These are structural, and have little or no impact on the sound of violin/viola/cello.  As long as you don't take out so much wood as to weaken things too much, they're a good place to hollow out spaces for the electronics. If the electronics are very small, you can put them in with the 9-volt battery (or explore some of the more modern battery types that are smaller - you may not need as much as 9 volts). Probably, you'll want the battery in one block, the electronics and jack in another, and the volume and tone controls each in one of their own. You'll want the controls to be on the side you're bowing from. (See my comments below on options for holding the instrument.)

4) design ideas: I'd like to add some detail to the scroll for any violin I make just as a bit of a maker's signature.  Is that taboo, or not recommended...? And what are some ideas? Also, I like purple, I'd like this to be noticeably purple, but still professional/suave/demure enough to be used in concert, i.e. not *too* flashy.  Maybe just purple on more easily removed parts like pegs and chin rest and tailpiece.  What about engraving or fancy inlaid wood on the edge...? I want this to be unique and beautiful, but not overbearingly flashy.  So what are your favorite designs? (please post pictures of your favorite violins to look at.

It's going to go from the bottom string of a cello to the top string of a violin, and be purple. Anyone wedded to strict tradition will have run screaming long since. You can do the scroll however you like.   In fact, I'd recommend drawing design elements from violas (and cellos, and violins), but it's not going to be any of those instruments, and trying to force it to be any of them is not likely to improve it. Let it be what its nature demands that it be, and then decorate it as you see fit.

1) what are some things I am unlikely to think of when modifying designs of traditional violas for this project? (like wider fingerboard, more bass beams needed?, larger than 16" better for sound?.....)

The fingerboard issue is going to be hairy. Make it too narrow or too large a radius of curvature, and the bowing gets impossible. Make it too wide or too small a radius of curvature, and the fingering gets impossible.  You might want to consider playing it like a cello. Most people have more difficulty playing the bass-most string on their instrument. Between needing to twist the hand and harm more, and reach further with the fingers, that string that is furthest away in the violin/viola position is tough. With an 8-string, it's going to be significantly harder. If it's played in cello position, though, all the horrible ergonomics go away. It's still not as easy to play the furthest string, but it's much easier that it would be in viola style.

There's also some reason for considering going to full-blown cello (or at least something like a 3/4 cello). An article I read on the acoustics of violins/violas/cellos mentioned (among many other things) that wolf tones are caused by the resonance of the body with one of the notes. They happen because violas, and especially cellos, are too short.  A standard violin is 14 inches. A viola is a perfect fifth below a violin.  A perfect fifth is a 3/2 ratio, so a viola needs to be 3/2s the size of violin, or 21 inches.  They make 21 inch violas, but they're very rare, because very very few people can play one that large.  A cello is an octave below a viola (2 to 1). So a cello should have a 42" body.  A full-size cello actually has a 30" body. 

If you make this beastie with, say, a 16" body, it's going to be 16/42 as large as it ought to be, acoustically. You're not going to have a wolf note or two, you're going to have a whole pack.  The bigger you make it, the better. And the bigger you make it, the more it makes sense to play it cello style.

I've seen a couple of videos of a cello-like instrument with 7 or 8 strings. Don't remember any mention being made of a special name for it, though.

If you DO make this viola-sized, octave strings are going to be an absolute necessity. They're expensive.  (They're basically the length of regular viola strings, but heavier, so that under reasonable tension, they'll play an octave below normal ones. Obviously, if you go for a more cello-sized instrument, you can use cello strings.) Since wolf tones are an issue of resonance at a frequency, I'd assume you get those with octave strings just like you do the more normal-sized deep-toned strings.

I'd VERY strongly recommend using traditional building techinques (hide glue, especially). One reason those have lasted as long as they have is that they allow you to completely disassemble the instrument, make modifications, and put it all back together. (Or less drastic subsets of that.)  I think you're going to be doing a lot of that. For a viola, you can find lots of info on how to do it, including info on variations on the main theme.  For something like this, a great deal of it you're going to have to figure out by experimentation.

For that reason, I'd give serious consideration to forgetting about good wood until instrument #3 or 4 (and maybe later than that). The issue you mentioned about bass bars is a good example. I suspect you're going to have one bigger bass bar - but multiple ones might very well work better. I suspect the only way to tell will be to try various options.

I've heard that around World War II they made cellos out of plywood. They were student grade instruments, and nobody brags about how wonderful they sound, but they were playable.  If you want a notch up from that, take two or three kiln-dried 1x6s, glue them together, rout them out to close the thickness you're going to want, then hand finish them. After you've finalized your design and worked the bugs out of it, THEN spend several hundred bucks on good wood and build one that's professional-grade. And the larger you decide to make the thing, the more strongly I advise that. A cello's worth of even 10-year old wood will probably run you $500-600, with you having to do all the carving on it.

5) what else should I be asking?

I don't know anywhere near enough about it to answer with any authority, but here's a few things I'd want to know if I were doing it:

* What makes different frequencies resonate. Are there shapes that will let a wide variety of frequencies resonate, or a group of shapes, one for each range? (I've seen one instrument that had several "bulbs" coming off of it, that I think were intended for that purpose. No information on how well it did or didn't work.)

* String instruments as we currently know them were optimized for volume. With electronic amplification in the mix, that's not really needed. Could you get better tone (across a wider range of frequencies) by sacrificing some of that volume?

* Part of the difficulty in doing eight strings is having them all the same length across such a wide range of pitches. Could a slanted nut and/or bridge, or staggered nuts be viable?  It'd take a whole new way of playing, but playing the full range from the bottom of the cello to the top of the violin on 8 strings will be quite non-standard, too. To make it playable, you might need to have the pegs all be at the top, even though the nut was much further down. Pegs in the middle would very much get in the way.

* Here's one you definitely should be asking: Has anybody already done this? I know I've seen cello-like instruments with at least 6 strings, and possibly as many as 8.

Ok, I think that's enough brainstorming for one day. Hope it helps.

That the smaller size doesn't negatively affect the tone of the lower notes much is a pleasant surprise.

I just had a thought. A number of things (like the tone, wolf notes, etc) can be checked on one of the regular violas you're talking about building as intermediate steps. It's the number of notes that's going to be problematic for resonance, not the number of strings. Test it with the octave strings, then test it with viola strings. That should give you an idea what it will sound like across the entire range.

Things like string spacing can be tested without bothering with a full instrument. A piece of pine for a neck and something wide enough for a bridge should be enough to check to see if it can be bowed at that spacing and radius of curvature. (I'm assuming you have a lathe for making fake necks and a belt grinder for fake bridges.) One consideration. If you make the strings close together, in addition to needing a smaller radius of curvature to make bowing possible, you're going to need a lower action to keep the finger-strength requirements from getting out of hand. It's going to be bad enough that you're playing viola strings, and octave viola strings. If you're having to depress two or three at once, that's going to get rough.

If you go for wider spacing, consider going with an asymmetrical neck, one that does not curve down as much on the bass side as it does on the treble. You simply can't get your hand in a position to point the fingers 90 degrees to a sharply curved neck if the curve is away from you. As far as the left hand is concerned, the bass side should actually curve UP. That won't work too well for bowing, of course. But with 8 strings, I think trying to make the whole thing symmetrical is going to make it very difficult to play.  (Of course, as I remember, you didn't like 9 strings because of the perceived asymmetry, which means this idea may be as popular as a flamethrower at a matchstick houses competition... )

Ooog. Nasty thought. You'll have to probably custom make your own tailpieces. Not too many 8-string tailpieces around, I suspect. Could you put together two four-string ones, I wonder?...  If not, I hope you're good at (and enjoy) woodcarving... I suspect a couple of normal 4-string ones could be joined with wooden pieces and work pretty well. Only downside might be that there might not be enough room to do it on the underside, where it would look decent.

I wouldn't think the tailpiece itself would be an issue. The tensile strength of wood is fairly high, and I think the tension on most strings is around 40-50 pounds. Even if it's up around 100 pounds (and behind the bridge it might be, those seem to under much higher tension than the sections in front of the bridge) a quick lookup of tensile strength of wood seems to be around 30,000 psi.  You don't have a full square inch, but I doubt that tensile tearing of the tailpiece will be an issue.

Crushing of the surface or the underlying block at the tail is a possibility, although I suspect it would be more cosmetic than structural. If I recall correctly, you normally don't have the tailpiece itself make contact there, you have the wire (I forget its proper name). Maybe a rounded metal piece to distribute the load some?  I think the part I'd be most concerned about would be the end pin and the block holding it.

From what I saw in the article I skimmed, it seemed that in general, the harder the wood, the better it was at resisting both compression and shear. So maple for the end block and ebony or rosewood for the end pin would probably hold up fine.  If you had issues (or wanted to soothe the paranoia a bit), make the hole bigger, put a metal collar in it, and make a somewhat outsized endpin. Or maybe even go with a CF endpin. I doubt you could break one of those. That would also eliminate any concerns about the tailpiece, but I kind of doubt you have the wherewithal to construct your own CF tailpiece.