Try the MIMF, Musical Instrument Makers Forum. They have a knowledge base of instrument specs.
Only two suggestions come to mind:
1. Keep it very light for good sound.
2. Don't use an armadillo - the wood ones are lighter and sound better.
Hi Steve. As you dug out my long ago started but never finished charango build blog on my "page" here on ferts.net and were so kind to contact me, I believe it would be just fair from my side to at least wetten my toes a bit regarding provision of charango building hints on this forum.
I have only built two charangos and a few other plucked instruments so far, but have repaired dozens of them over the last 15 years or so.
As wood species goes it actually doesn't matter much which hardwood you use.
In Bolivia, even if some luthiers never would admit, they like to work with naranjillo because it is so easy to work with: it is uniformly dense, reasonably hard and yet soft to cut and carve with virtually any tool, it is mainly knot-free, not splintering, you don't have to worry a lot about the rdying process, it's actually a dream to work with. But on the other hand there are also a lot of Bolivian, Peruvian, Argentinian and Chilenian luthiers that use just about any kind of hardwood you can imagine, and those charangos all sound wonderful and hold up well if correctly dimensioned and built.
I recommend you to build a two-piece charango, that would be one piece for the body and another for the neck. I have built both, a two- and a one-piece charango.
The reason I'm inclined to recommend a two-piece charango is getting more control over wood quality. More often than not, in a bigger block of wood there are hidden unpleasant surprises such as changing grain direction, worm holes or little knots.
Also, when using two pieces you can set a neck angle which allows you to avoid an in my eyes unaestetic overly wedged fingerboard. And last but not least you waste much less wood compared to making a one-piece charango. There are even charango luthiers who join several plank pieces in order to make a block big enough for the body. Depth of the body should be somewhere from 80 to 90 mm if the lower bout is 180-190mm wide.
Common string length for a charango is 370 mm, but also 360 mm, 380 mm or something in betwee are often used. 390 mm and larger can get a problem regarding the strings: some firsts will break too soon when tuned to standard tuning, and some string brands simply will be too short. I see no benefits sound-wise for using a larger string length than 380, you only would add problems. On Bolivian charangos, very often the body outline joins the neck at the 13th fret, but there's no rule to it. There are also many charangos which join the neck at the 14th fret or even more up, less often you also find them joun the neck at the 12th fret or below, or even just somewhere between. Again, there are no set rules.
Proportions of the soundboard and soundhole are best those of the classical guitar or also the romantic guitar of the 19th century. Many younger charanho luthiers now rather use soundboard- and soundhole-proportions of the steel string guitar world, which for a charango looks wrong to me, but hey, it's just a matter of taste!
Ideal string action (12th fret) is 2.5mm, but if you can get it down to 2.2 or even 2.0mm that will be just perfect. I regard 2.8mm as the absolute maximum for comfortable playing, 3.0mm is definitively too much for me.
Neck width at the nut is around 48mm, 46 is very narrow and 50mm would be extremely wide.Fingerboard width at the 12th fret is (for a 48mm nut) 59mm.
String-spacing for a 48mm nut is 3mm within a string pair and 7.0mm between string pairs, leaving 2.5mm on either outer side.
String compensation on the bridge/saddle is string brand and make dependent. Using a saddle, calculating about 2 mm for the tickest string (low E of 3rd string pair) and an absolute minimum of about 0.5mm for the high E-strings should be fine for mosts strings available on the market. Making the saddle (slot) wide enough (ideally 3mm) allows room for variation.
I hope I could help a bit. If I missed a question or if you have more of them, just ask me.