I never realized until this thread popped back up that I didn't answer this question. Somehow I missed it at the time. Anyway, I actually didn't have the 'proper' materials to replicate it (body schutz, as breakstuff mentioned above), and the customer really didn't care if it had anything on it. But I used a few coats of spray can bedliner material to build the area up a little, then used paintable undercoating over that to somewhat duplicate the original texture. It actually looked pretty good.
Dallas