A very cheap lightweight plastic shoulder rest came "free" with my acoustic violin.  I don't like using it with the acoustic, since the instrument honestly just balances better if I just let it rest on the top of my shoulder and collarbone.  The acoustic is so light anyway, that I rather prefer playing it without the shoulder rest as well as without a chin rest.  Other than being comfortable, I also pick up a fair amount of the sound via bone conduction that way, and it is just cool.  It is light enough and shaped just right or something, since I can shift positions on the neck and etc easy without a chinrest.

My electric, though, needs both the chinrest and a shoulder rest to be able to play it.  It weighs probably 3 times as much as the acoustic and the shape and balance are very different.  I ended up making a shoulder rest for it a while back, because when I took it to the music store, we tried every rest they had (which was, like, five), and none of them would hold it right (some wouldn't open up wide enough to even go on) and they told me that no standard shoulder rest was going to fit that.  It has no "rim" for a shoulder rest to hold on to, and it is thinner than acoustic violins and since the curves are so different, one wouldn't expect a standard rest to fit it.

Just for the heck of it a few nights ago, I tried the cheap little plastic rest that came with the acoustic on it.  Of course, with the perverse nature of the universe, it fits.  Tried the test they did at the music store of seeing if I could hold the violin upside down by just the shoulder rest without it coming loose.  Even gave it a little shake.  Holds fine.  Ok, then. 

It is considerable lower than the one I made, but also less bulky and it allows some air flow, which is nice considering weather has been pretty warm here lately.

But now I had the problem some folks I guess usually run into.  The metal bottom of the chinrest clamp kept finding just the right angle to dig a corner into the top of my collarbone.  So I tried the cosmetic sponge thing that FM uses.  But the thinnest one in the package was still about a half inch thick and it felt too thick and "mushy" for my liking.  Then I saw a little foam "tire" from a defunct set of cheap headphone I had lying around.  I thought it would be too small or thin, but nope.. Perfect.  A few minutes to find a way to rig a rubber band to hold it on and we're set. 

I've been playing with it this way for about 3 days now, and it stays in place well and is comfortable.  Even better, I doubt the average person in an audience would even notice it was there.  Since my electric came with a gig bag rather than a case (the shape is too odd to fit in a regular case made to fit  violin anyway) I don't actually need to take off the shoulder rest to put it away.  Go figure.  Oh well, at least the cheapo shoulder rest I got with the acoustic is good for something.

But if you find FM's cosmetic sponge pad trick a little too thick for your liking, you might want to try the "tire" off a set of headphones.