1 rule for box section welding, closed sections are a lot stronger than open sections. So, angled at 45° and welded then or just welded together and then the ends welded closed wont make a great difference.
But I garanty you that the box section you say you will dent and rip under the power of the machines you name.

On HDPE, 50mm HDPE weights as much as 6mm hardox or 10mm Ti. 20mm is more than enough to protect against all but hyraulic crushers and the better disks around. Ald layering a thin sheet of ali or mild steel over it, pointless.