I am using 3/4" sandeply from the Home Depot. Very strong, but I do know that it will probably absorb water in the future. The other wood was 22 years old I think. I am going to cut the edges of the new stuff at a 45 degree angle.

I will coat it with resin, lay in place, then resin again, cover with fiberglass cloth (rough matte) reapply resin.

Might do two layers of cloth with resin between and on top of??

Do I need to clean up the entire underside of this hatch (corners and edges) or is the center area enough.

No use of grinder. Wish I did.