I built a table for a 400 lb. mill out of 1/32" 1X1 steel tubing last summer. Yes, I used a lot of cross braces. But it was both light and strong.
The only reason I used steel was because I already had it, and didn't want to spend the money on aluminum.
As for aluminum C-channel. I don't think it's that hard to find. We always get some 1/8" wall 1" interior stuff that we can hammer 1x1" tubing into. By doing this we don't have to worry about clamping the frame when we weld it. This saves time, and clamps...
