We use a material called verolite which is what greenhouses are made out of, it is essentially corrugated polycarbonate. We choose this material for several reasons
.5 inches thick (roughly) means that despite being incredibly light, it has excellent rigidity.
2.17 per square foot from our supplier makes it substantially cheaper than other materials.
It is non conductive, machinable with wood shop tools, doesn't splinter or crack(it is spry enough that it would take an enormous amount of force to reach its ultimate yield point.
our dual belly pans(one on top of the other) weigh < .1 lbs each, and the 15mm thick variety of verolite that we use is stronger than half inch thick particle board by far, and although it flexes more it has higher ultimate yield properties than almost any type of plywood.
This year almost 2 full 4*12 sheets made it onto the robot.
http://www.emplastic.com/images/library/BC_Twinwall.jpg