I want to experiment with the doomwheels and my kitewing. So I am trying to keep this cheap, as in whatever is free as possible. I can use some 3/4"x3/4" square steel tubing for free from work. As I read weight is good for speed and stability, I was thinking this tubing with a gusset would probably be rigid enough but I wanted to get some experienced opinions first if possible.

I also think we have 1"x3/8" lying around somewhere too. Would this work, as long as I maintain the 3/8" width?

I tried the square tube once, but bent the rails just behind the heel. The boot mount holes weaken them too much. I'd stick with solid stock if you got it.

cool, thanks for that. But about the 3/8" width. I can't seem to source (free mind you) 3/4"x3/8". Everyone I talk to says it's an odd size. So I can get it seems 1"x3/8" or 3/4"x1/2" solid stock. I was thinking maybe 1" might be stretching the profile of the steel a little thin. I can't really see any reason why it can't be an 1/8" wider with the 3/4"x1/2". Should still be pretty rigid (that's what I am assuming I am after since you are using rectangular solid stock instead of square solid stock)
So I think I'll try the 3/4"x1/2" and if I have any issues I'll just have to buy (shock,gasp,screech!) some 3/8 width.