[EricH]

Originally Posted by Ether

I don't have a link handy, but I recall seeing a recommendation to use thin plywood with fiberglass cloth bonded to it with resin.

Originally Posted by JesseK

I remember that too, somewhere. It might have been Baltic Birch though, which is a bit lighter than plywood and commonly comes in multiples of 1/4" thickness.

Just a couple of days ago, too!

Originally Posted by Gdeaver

An alternative to the 1/2" plywood would be to take a piece of 6mm or 1/4" birch plywood and laminate a layer of 5 - 6 OZ carbon fiber to it. You'll get a very ridged and impact resistant material. We do this for our electronics board and other parts. The goal being to introduce the students to some composite construction. You could make your own honey comb stuff but it is more difficult than the plywood and carbon.

This is in the thread discussing honeycomb material.

330's used various bellypan material; I believe we used 1/4" Baltic Birch in 2005 and built areas that weren't running under structural frame members up to 1/2" with glue. In 2007, using plywood, a pattern was traced on the bottom to allow electronics to be supported and have extra material around screws, then the whole thing was routed out. From the top, it looked solid because we didn't go all the way through.