Another roll pan MAY work, but finding one with the proper shape and curves may not be easy, and probably pricy.
You can make a foam form, shape it the way you want, then lay fiberglass over it to get the desired shape, then you'll mold it in as you normally would with a plastic bed.
Structure wise, you could make some simple brackets that would bolt to the fram, and if it's a metal roll pan base, weld the brackets on, or fiberglass them in if it's an all FBG pan.
Even though this site hasn't been updated in over a year, it shows the process of making a foam form then fiberglassing over it to make a roll pan for an Explorer Sport...<a href="http://stlouismtb.tripod.com/rollpan/rollpanpics.html" target="top">linky winky</a> EDIT, on this site it looks like they made a foam shape, then made a mold from the foam out of fiberglass, then used more fiberglass to make the actual roll pan.