I made them in two pieces so I could get the radius on the side of the box and the corner of the bed then welded them together.
The bedside I bent over a rounded peice of scrap I had laying around, for the corners I cut perforations with my plasma, bent the radius, welded it up,shaped it with a grinder then polished it smooth with a sander.
I now have a slip roll machine so assuming the pieces fit, (I haven't measured) I would go that route if I did it again.