You could also mold a metal one to the fiberglas, right? Of course, one of the reasons to use a fuel door on these fenders is to make the filler tube "go away". If the fuel door is too large, it will probably have the opposite effect and draw more attention.
On these fuel door kits, the surface sheet metal is tack welded to the box. It would be easy enough to remove the surface metal and mold the box to the underside of your fender and cut a hole in the fiberglas to use the metal fuel door. It would be small and match the contour of your fender, has a spring-loaded magnetic catch and should work fine. I installed one on my '37 fender (steel) but I welded it in. Just exploring options....
prd_lg_2266fuel door.jpg