Technically, you don't "cast" in fiberglass or carbon, it is called a "layup". But that aside, don't bother with carbon fiber unless you have very good skills in "maximum cloth, minimum resin" lay ups, or you plan to vacuum bag your lay up. Carbon fiber is one of those subjects that people talk about and either leave out important facts or in most cases don't know them. Carbon is lighter than glass fiber, but as was said, your strength comes from the fiber, not the resin and without vacuum bagging the layup, to "press out" excess resin, your resin to fiber ration is not sufficiently low (resin) to give you a significant difference in weight to strength. Also, you want to use epoxy resin with carbon fiber, not polyester.
I have never used pepakura in it's "formal technique", meaning using a computer to make a polygon version of an object and create a card stock or paper model. But, over the years I have many times made card stock or foamcore "molds" for an item in order to quickly and accurately prototype an object in fiberglass.
Something you might try: Instead of soaking the paper with resin and adding material in to, and on to your paper model, try making your pepakura model with card stock (like business card thickness). Tape up model on the outside, not worrying about the thickness of tape.
Then using a solvent based wax or PVA (polyvinyl alcohol) coat the inside of your pepakura model. The liquid PVA or solvent based wax should be thin enough so that it soaks into the paper/ card stock. (it may require two coats depending on what your release is)
You now essentially have a mold. If you mix auto-body filler (Bondo) with polyester resin, until it is a pour-able consistency, you can coat the inside of you model with an 1/8" layer of this sandable "gel coat". Then, lay up glass fiber with polyester resin inside of this bondo/resing surface coat.
When it's cured, peal off the paper and you have a outer surface that is as smooth at the paper and you only need sand off the corners. If you need to build up any areas, the body filler will adhere great to the Bondo/resin surface coat after a mild sanding. This technique allows you to control the outer surface of your part much better, thus requiring far less cleanup and better accuracy to you original pepakura pattern.
