Very nice! I'd suggest making the front and rear crossmembers go past the wheels, so you can attach bumpers to them, and add blocks between the wheels to support the middles of the side bumpers.
shows kind of what I mean...although we added a vertical piece of 1/8" x 1-1/2" alumimum angle at each corner later to support the ends of the side bumpers.
Also I am not a big fan of using a single material for the entire robot, it seems to me that it's wise to select the best material for each part (within the constraints of your fabrication abilities and budget).