[MrForbes]

We usually let the chassis be heavy, so we are forced to make the rest of the robot light. And we like using a variety of materials. Thin aluminum tube, polycarbonate sheet, thin aluminum gussets, thin plywood, etc. If you do it right, you can have some weight budget left over, and maybe put a couple wood 2x4s in the bottom of the robot, to impress the judges.