I'm building the $50 robot, but I didn't want to use cardboard, and the only HDPE I have laying around is a bucket of kitty litter. I was in a hardware store the other day and found some great stuff real cheap in the electrical building dept. I'm making my chassis out of a $0.25 electrical box and the $0.45 blank cover. I've gotten it put together so far with everything but the servos. The circuit, 9V, and 3AA battery pack all fit mounted on the cover, and there are convenient grooves to place the sensors into. I'll upload a completed picture when I get the chance.