Word up for the Baltic Birch ply wood. Our team (195) has been using 9 ply for the last 4 years and its worked pretty well for us. What we usually do is build the box and paint it and then screw plates of 1/8 inch lexan around the outside as protection against dents, alot of times we will also reinforce the front and back with some 1/8 inch angle aluminum because in some extreme cases the frame has cracked on us. The aluminum angles seems to aliviate this. Our last bit of "armor" is 4 upright pieces of 3/16 inchangle aluminum at the corners to guard against spiliting. This has proven to be very structurally sound and also presents a good base for mounting things; blind nuts for important things, machine screws threaded into wood for relays, speed controls, etc.
P.S. Shameless plug for Gorilla Glue.

Works awesome on our frame.