MUST be dry (not moist) or it will rot further like boat cancer. A bad repair just doesn't go away over time.

If you slap it together like you're planning it'll be OK for a year or two, maybe three, but if you don't do a proper job the long-term prognosis is more rot and delamination.

If you plan to keep the boat long-term -- do it from the outside if you can't access the inside. Grind it out, re-glass and spray gelcoat. Hire a fiberglass guy if you don't think you can do a good job -- spend a boat buck two to preserve the integrity of the boat. Maybe the outside gelcoat won't match exactly, but a proper repair that's detectable is better than a crappy one that isn't.