My recommendation is to use 1-1/4" steel pipe, depending on how much you’ll have on it. When loaded, aluminum can eventually bend. To prevent water from getting inside, plug the ends.
My entire gym is made of scaffolding.
The freedom to create what I need is something I adore. I’m still moving forward after being fortunate enough to find a ton of scaffolding and fittings at a low cost. I use the barbell with fixed connectors. It’s difficult to describe, and I’m not sure how to include pictures here