Hi @Mouse and others.  The sound post is projected through the hole in the belly.  Not sure of the construction of the bridge “dowel.”  I would think that you would want to use something strong and light to avoid inertia (like a titanium or carbon fiber pin).  It might also be possible to joint the sound post and bridge together and secure with glue.

My assumption is that the treble frequencies are relegated to the less resonate back plate and the bass frequencies favored on the belly plate.  

Harbor Freight makes an automotive spot weld removal cutter that would be perfect for the heart hole.  A hollow diamond hole cutter might also work without splintering the back side of the plate.

Of course, I would also reduce the size of the tail piece.

Hi @bocaholly and others.  Since the bridge is in direct contact with the sound post, incremental adjustment really has no relevance here.  

Hi @@bocaholly and others.  Sound post placement normally has great importance for the resonance of the top plate.  The whole purpose of this contrivance is to divorce the top plate from amplifying the output of the treble strings.  The bass string component is unaffected.  The acoustic balance of the instrument is therefore skewed toward the bass.  Foreward and aft bridge positioning would change string length and is prevented by the size of the sound post hole.

This admitted butchery is restricted to low cost instruments in small sizes to provide a flavor of the sound of a proper viola.  

Hi @@Gordon Stumway and others.  Fractional violins are notoriously bright creatures and this is a low cost method of mellowing them out.  Obviously, it does nothing to increase air volume in the corpus (the Sabatier fractional viola will do that, but costs a good deal more money).

@bocaholly, most of my instruments were obtained inexpensively and I could easily do this, but I lack anything bright enough to make it potentially worth while.  If I had a grand child that wanted to play the viola, I would either buy them a Sabatier and/or make them a clone of one.  

@Pete_Violin, you are of course entitled to an opinion, but I see no logic is spending $150 to a luthier to modify a $40 instrument (assuming an eBay purchase for said instrument, of course).  I see it as a potential bonding moment between child and adult.  Using the provided photographs, this procedure is easier than righting a fallen sound post.

Hi @Pete_Violin.  Absolutely no offense taken by your comment.  We simply see the violin under different perspectives.

You see a object of art that reached the pinnacle of perfection 300 years ago and has been in retrograde ever since.  I see a slightly sub-optimum but beautiful tool subject to modification to suit the user.  

The experience and training of a willing luthier would always be an advantage, if available.  It is a pity that their voice is not evident on this forum.  Perhaps @Fiddlerman should avail his luthiers for such a voice.

My best experience viewing a post on the forum was a video of Maxim Vengerov (a player previously unknown to me).  While I immensely enjoyed the sound of his playing, I also enjoyed cataloging the various supplements he had made to the basic instrument to assist his technique.

I doubt that any members of the forum will subject an instrument to a hole in the top plate, but a guest might.  And the guest might be the parent of a future Maxim.  

Further back in the thread, someone mentioned the dowel connection between the bridge and the sound post.  I did some browsing at it appears that a ground off piece of sewing machine needle is a suitable material.  2mm and 3mm titanium rod can also be obtained inexpensively from China (5 each of 10 inch length for less than $3 inclusive of shipping).

I also read that the belly hole needs to be oversized, particularly when dealing with young students, to prevent a buzz from occurring if the sound post comes in contact with the belly plate.

For those with an interest, check out the December 2000 Strad Magazine article entitled:  “Too Small to be Beautiful?” starting on page 1350.  The “dark horse” 60£ Chinese fractional violins with the 35£ hole in the heart modification kicked butt against Sabatier amoeba-shaped violas and Tertis viola clones (among others) in a blind sound test with 60 music teachers and makers.  All entrants used donated Corelli Crystal strings.