I went to my local hobby store and bought a sheet of thin balsa. Temporarily installed (cleco) the fiberglass tip in place, and cut/sand a piece of balsa to fit. Tacked it in place with a few drops of CA adhesive (super glue), and it was ready to lay a couple layers of fiberglass over it.
I fretted over this too, but it's actually a lot easier and quicker than the manual, and these instructions, make it sound. If this is your first go with fiberglass, then just dive right in. There's nothing you can possibly mess up that cannot be fixed with sandpaper, more fiberglass/epoxy, and some time.