You can get aluminium watercut quite cheaply.

Unless there's a huge shell to go on this, a 10mm end-tapped (meaning no finger joints or anything) aluminium box structure doesn't sound very sturdy as the exterior of the robot. It would be fine for a regular style machine, but one which is spinning is going to attempt to skew the chassis; squares are inherently weak in the direction of forces this shape would suffer.

Do you intend to attach teeth directly to this chassis or is there a shell to go around that just hasn't been designed?