Ron - regarding the problem with your screw heads digging into the balsa, try drilling out the existing thru holes to the dia of your screwheads and then epoxy in hard wood dowels of the appropriate thickness, drill your screw clearance holes thru the dowels and it should beef it up and won't show indentations. You could model it in Solidworks first if you want
. If the dowels should ever fail, you've probably got bigger issues at hand. Just a thought.
Todd
WACO Brotherhood #47