Does the top of the roll pan have a lip on it that would be against the bed floor?..If so, get out the drill and that is one way it can be mounted. You could use fiberglass on the back side of it to mold it in, but if you're not planning on painting much, you might not want to do that. I have a steel on that I only have bolted to the bed floor,.. It would be molded, but the company, FBI, has REALLY slow shipping, and the pan came in several weeks after the truck was out of the paint booth, then I realized i was slightly limited on how to mount it.
Anyways, it can be done, even if it takes some creativity!
goodluck