I have been working with pepakura and have currently done a number of items. I did a Boushh helmet & a number of other items, http://www.thedentedhelmet.com/f68/boushh-wip-47893/. This costume is for my wife and is almost completed.
I used 110 lb. card stock & reinforced that with .060" architectural cardboard. In some cases I went with 2 layers of the cardboard. When I completed this I used smooth cast 320 & coated inside & out. This filled in all the cracks & voids and tied everything together very well. Then I proceeded to do all of the filling and smoothing things out. I used wood glue to glue everything together. It creates a great bond. What my plan was with this method was to create a lamination similar to plywood. With the card board core I don't expect any warping. I wanted to try something other than fiberglass.
I am also working on a ROTJ Boba Fett as well for myself. But not as far along. I did the jetpack in pepakura using the above technique, http://www.thedentedhelmet.com/f20/pepakura-jetpack-47919/ (Skip & Soba). I have helmet in the assembled & in process as well (Skip) with some modifications from WOF & Rafal's templates.