Doesn't fiberglass laminating require special molds and so on? I can imagine this would be ridiculously expensive if that were the case, especially given the time it would take and labor that would be involved in trying to replicate (as much as possible) the handguards that are shown in the above photos.
If you look inside the forearms mentioned, you'll see the fiberglass resin and cloth are laminated over/on top of the wood's surface. The actual thickness added is quite small. The "special molds and so on" are needed only for laminating layers of wood, etc., as seen with laminated and/or composite stocks, not for the reinforcing as suggested.
I've never done this on a firearm, but I have used this method to refurbish/repair both old wooden strip-built and fiberglass canoes.