Suggestions for sagging floor gas tank access panel - Seafari 20’

[Bushwacker]

I would seriously consider using end grain balsa instead of plywood. It will save a lot of weight, is cheaper than Corecell, will bond to the fiberglass better, and if you do happen to get a water leak in one spot it won't follow the grain and spread everywhere like it will in plywood. You might also consider using vinyl ester resin, which will be a bit cheaper but with properties almost as good as epoxy.