Saddle on Soundboard

Hello folks, I am getting ready to make a dulcimer for myself and would like to make it in the style where the fingerboard stops after the second octave, a saddle is sitting directly on the soundboard, and the strings are anchored to a tailpiece. (not sure what this is called) I would like this dulcimer to be generally hourglass shaped (without curved sides) and would like it to be at least 8" across where the saddle will be.I have 5mm plywood that I would like to use for this, but I am concerned about the structural integrity. My two biggest concerns are that when string tension is applied, the entire dulcimer will bow at the point where the fingerboard ends, since that is where the bracing ends, or that the bridge will either crack, deform, or punch through the top board. I dont know if any of this makes sense but hopefully these bad drawings will help clarify what im worried will happen.
I've never done anything like this before and I'd like to avoid an instrument exploding on me (hasnt happened yet ) so what Id like to know is how realistic is this concern, and also how much extra bracing they need. I dont want to give up all the extra tone and volume I'm hoping to get, by putting an excessive amount of bracing in just to keep my paranoia away. 
As always  thanks a ton for all the help. I'm not a luthier or even much of a hobby woodworker and it's still very easy to remember how far in over my head I was before I found this place.
Nate