If you don't have access to waterjet/CNC facilities, you could just stick a thin aluminum plate on the bottom. The NERDS used .060" Aluminum for a bellypan in
2008; nice and sturdy with lots of real estate to attach stuff.
1/4" plywood works wonders, lightweight and cheap! currently my material of choice for something like an electronics board/bellypan. We're sporting quite a bit of plywood on the Falcons' robot this year.
We played with some mystical Boeing honeycomb composite material this season, glued between two sheets of paper (yes, like printer paper). Extremely light and stiff. The plywood was faster and easier to use though, so it won out. Maybe next year...
