Square steel tubing question

For using square steel tubing to make up a robot frame, what size should I be looking at for a featherweight to stay within weight? I'm guessing 1/2 inch? They do square ally stock as well but doesn't seem any where near as solid but its loads lighter, could I get away with this?

For feathers I used steel box section of 20*40*2 and 30*30*2, some drilled full of holes, some not.

Ali box section 30*30*3 wasn't fully satisfying in Hannibalito4, so that got serious reinforcement with welded on 5mm thick bulkheads.