You could likely add trespassing to the theft, but it's all good now. Nice to see someone with a conscience.

I would not have just taken the material, you could have gotten into a lot of trouble over a pile of crap-quality aluminum that might only cost $30 at a hardware store.

But anyway--I would ask, are you familiar with welding aluminum? It is much more difficult than steel, and the aluminum requires heat-treating afterward, as it softens around the weld area.

If you don't know anything about welding then I would suggest you draw up plans for the tubing you need, and then order 4130 steel from somewhere that sells pieces by the foot.
Steel is much easier to braze (not weld!) and any welder you get your hands on will work on regular steel. Most electric welders don't work at all on aluminum.