Get out the marking pen and cardboard (sveral boxes flattened and taped together should do) and make a pattern of the rear curvature of the existing sheet metal (key word--existing).
Use this pattern to check automotive or truck/van panels and see what you come up with.
You never know until you try.
I'm no expert at this by any meaning of the word, this is my first try at something like this but I'm making a rollpan for my '46 Chevy panel truck now, if you can wait a day or so I'll be able to post some pics and tell you how I did it.
I'm about half done right now. So far I made a pattern, got some 20 ga. sheet metal, cut it over size to work with, started the sharp bottom bend over a peice of 3/4" pipe, started the bigger curve between two boards with a mallet. It doesn't sound like much but I think pics will help.
So far its comming out pretty good. I'll let you know more when I finish.