To hold 10 pounds you don't need a 2x4 Chassis. It will add unnecessary weight to the thing an deplete your batteries faster when moving.Alluminum will be ideal; but probably too expensive. Simple 1/2"x 2'' or 3/4"x2" boards will probably work fine for the frame at a fraction of the weight and cost. You can just reinforce the corner joints with triangular pieces of plywood.
So my question is, what's the best material for a chassis, 2x4 axle with plywood on top? plexiglass? any suggestions?