I wouldn't want to run without a box.
Aluminum is always a chore to try to bend and weld.
Tupperware is cheap and easy
But, if you can't find something that works Louis, I think sheet metal would be the easiest thing to make it out of. Easy to bend and form, yet still light enough and everything can be braised (easy) or mig welded place, with brackets to mount it with and everything.
Maybe 22 gauge?? Just thinking here................