I have tried all kinds of bracing to improve box stability, and to see what sound comes out. I made a six string dulci-mandolin with very thin all wood box, with an X brace, and it sounded very good. I also put an X brace on the bottom with a "sound post" on the bass string side. I was surprised at the nice tone. The box was Cedar. I put kerfing along the inside edges to strengthen the box, as it had six strings with lots of tension, tuned in the mandolin range. I took it to a jam and sold it. That was the best sounding CB mando to date. I now make them with the stick on the top, like a traditional dulcimer.
Probably silly to resurrect this thread (on nearly its 5-year anniversary :) )
It occurs to me that one could achieve a thinner (and presumably higher resonance) CB soundboard if, rather than adding braces, one subtracted material (thickness) with a router and the material that you LEFT became (in effect) the bracing. I think this idea would work best if the lid is solid wood (not plywood). Would be interesting to compare two identically constructed CBGs with this "router reveal" of bracing being the only difference.