[sgreco]

You could consider a 1x1 bar of 80/20 insead of what you have for the cross pieces. The ends can be tapped for a 1/4 20 bolt and you can just bolt it to the outer plate. This also leaves you a lot of flexibilty on the inside as far as mounting other components.

I think it would work nicely in a prototype like this. My team actually used a very similar frame to this this year, aluminum side plates, cross beams extending to the outer plate, toughbox transmission etc...Anyway the 80/20 worked nicely for us on the cross braces.