The nice thing about the PVC foam is that it is shapable with a heat gun and some ingenuity. I've done this many times in the past.
But for my money, the simplest thing to do smaller repairs with is good ol' sheet balsa. I use 1/32" sheet usually, wet it and shape it in place and dry it with a heat gun. Then I apply the glass to both sides, as Shawn suggested.
The foam often has half the density of the balsa and is quite useful for large patches. But for small stuff, say 30 square inches or less, balsa is easier and the weight penalty is so small, it is negligible
The formability is exactly why i import it.. The fact its lighter and stronger than the same size piece of balsa makes it even nicer.
