Definately a scrapyard challenge

[Jonathan]

I thought about 80 x 40 with the rails top and bottom.

80x40x3 box section is less than a third as torsionally stiff as an 80x80x3 box section. Same reason as before - material is distributed further from the centroid with a square compared to a rectangle.

These photos show mine (old pictures!) with the size I mentioned. I put 20mm thick plate in the ends of the box section to make as strong joint.