Running Aluminess in the rear. I have quite a bit of weight on dual boxes and no cracks or fractures. If you mount boxes about 1/4 inch out (washers at the bumper and box mounts) from stock mounting position you can sneak rotopax units behind the swing arms. Aluminum is very thick but bumper with swing arms is light enough to lift with some muscles.
I have actually hung a hammock between the loaded open pinned boxes (tied to the rails) and no sag or shift. I'm about 300lbs.
If you just want a tough bumper the stock Ford hitch bumper was good and cheap to replace. I wanted the storage though.