Just fill in the six inch hole with a piece of marine
plywood, West system epoxied in. You will need to use an orbital sander and grind out a tapered "divot" twice that width in the fiberglass
and fill it with at least two layers of target shaped torn pieces of chopped strand mat. Then several applications of polyester fairing compound like Evercoat. Forget the Nicro, I took out two of those worthless pieces of garbage. Noisy, leaked, and very little air flow. If the hole is in your deck, it will show somewhat no matter what you do if there is intact non skid surrounding it.