I am planning to make a panel that's about 48" by 24". I'm planning to use square tube of some kind with 45 degree mitered corners to make a rectangular frame, and then rivet or tack on a sheet metal skin. It's for an artsy thing...kind of like a metal painting canvas. Very much like the frame in this video.

Anywho, the size of the box tubing isn't mission critical, nor is the type of metal. I have a Fireball Tools mega square on the way, so am feeling ok about getting it squared up initially, but my concern is is warping as I finish off the corner welds. I'll follow the video's advice on weld sequences, etc., but my question is about material selection and what is most/least likely to warp?

aluminum or mild steel?
.065 wall or 1/8"
1/2" box; 3/4"; 1"? Bigger?

All things equal, I'd probably use 1" aluminum box tubing with .065" walls, but am totally open to steel, bigger/smaller box and wall size. Advice?