Thanks for the replies. It sounds like it could actually turn out to be a bad idea to have the legs too strong (i.e. the material I currently have) because they'd damage the frame rather than perhaps bending out of the way.

I'm kind of biased towards square tube since there's no notching involved and therefore slightly easier to make joints.

It sounds like the best way to go for me would be .120 2x2 square tube for the whole thing, with some clearance (1/2-3/4") between the body and any support members or the 1x1 tube across the top.

I think I'll save the 2.5x2.5x3/16 for bumpers then.