there once was a post of how someone endoglossed (i think that was his custom term) his rifle to give it an internal frame. it might be more than you want to do right now and he never did a full tut, but he cut the pep in half and fiberglassed the inside and put it back together and bondo'ed the seams. looked pretty nice, add some resin, probably look pretty good. i have alot of experiance with expanding foam, not with modeling in specific, but you could end up doing the same thing to your pep that it did with your homemade mannequen