Afterall, I was going to install a 10x10 inch hatch to ventilate the head area anyway
And heck while I am replacing the fore hatch, might as well replace the aft one beneath the boom too.
"Heck, might as well" is how project inflation works.
Anyway, the fore hatch is a Lewmar 60 Ocean with a flange (not flat) bottom and it is just about a half-inch smaller than the existing hole in the foredeck, so fitting it in place won't be a big deal; I will just need to shim the sides with fiberglass and grind down the area on top of the existing hatch hole to remove the camber and make it flat.
The hatches are meant to screwed-down but I want at least 4 bolts that go clear through the deck with washers and cap-nuts on the other side. All the screw holes will have to be pre-drilled a larger size and filled with fiberglass too, to prevent water intrusion into the core around the hatch. Easy-peasy.