Two part foam... Buy it in bulk, I'm not sure how much though. Also clay, regular modeling clay. I bought water based when I was going to custom wide body my civic. Shlack to harden the clay. Fiberglass and resin. Pre-plan your design and mounting for the panels. Mix your foam and let it dry (following the directions on the foam). Sculpt the foam as close to the shape as you'd like. Go over the top of the foam with clay, covering the entire foam that will be fiberglassed on top of. Shlack the clay let it dry. Come back over with large sheets of fiberglass and resin, this takes time to lay the fiberglass flush with the panel. Fiberglass should dry in about 24 hrs. Remove the fiberglas from the mold that was made with the foam and clay, (this piece should be reusable if done right). Trim your excess fiberglass and make panel smooth by either using body filler or sanding fiberglass down, using 80 grit but finishing with 180 grit. body filler is not a bad thing unless used in large amounts in one area... If the body filler is thicker than 1/4 inch then its too thick. In this case use more fiberglass. Test fit all panels eing made to ensure fit and alignment. If all panels don't line up just right take your time and go back and more sanding and fiberglassing if needed. When done, preferably take it to a paint and body shop, a Gel coat is highly recomended but a fexible preimer can be used. Wet sand/ block the panel, if you don't know how to block at this point then your panel will be wavy. Then paint the panel once blocked wet with 600 grit. Three coats of clear will be all you need if done correctly even if you want to slick the panels out. Install and enjoy a complete fiberglass interior. More info on fiberglassing and mold making goto
http://www.chavant.com/index_main.shtml
It might have some more info with a bit of browsing on clay mold making. Using the foam is just a cheaper way than using all clay.
Or are you talking about just painting the interior with rock chip spray? Which IMO would look like A**. Rock chip spray is used on alot of factory cars to prevent rock chiping on lower parts of body panels. Just imagine a paint job with the most orange peal you had ever seen.