OK, I will expand this to a more general discussion of octave violins and fiddles.

I am presently making them by modifying violins. This requires removing the top, fitting an additional bass bar, reglueing the top, modifying the pegs, nut, bridge, and tailpiece.

I brought one to the FSGW Getaway, and sold it there.

The one maker of OVs in the US that I have found uses 14" violas, and does not appear to change the inside ( though it is not certain- he does not describe his mods specifically). There are two makers of OV strings, but all the discussion I have seen are that the SuperSensitives work better. ( Some comments were on unmodified violin bodies, though)

I am making several based on 14" violas, and having a friend ( violinist) try them out and compare to my prototype modified violin version.

The luthier mentioned by Micca ( in UK) makes the entire instrument, and it is priced appropriately for a custom instrument- but higher than I think many can go. I would like to see decent instruments in the $500 to $2000 range, as opposed to $3000 or $5000.
Parts ( wholesale ) are around $100, and it takes about 8 solid hours of work ( spread over several days waiting for the glue to set) to modify a violin ( plus any repairs the violin requires).

I am using older ( mostly prewar German) violins ( and violas) that have needed repairs to the tops ( so I had to open them up anyway). I suspect that the better instrument I start with, the better the tone and voice of the resultant OV, but have relatively limited funds, so...

Does anyone have experience with Octave Violins? Any feedback as to modified violin vs small viola vs custom in terms of the instrument "voice"?