Since Capital Yachts put frp hatches in the cockpit sole of hundreds of Newports, I would say to go ahead and design one out.
You'd likely need to reinforce the flange a lot.
For the new sole dover, use balsa or honeycomb or coring to keep the weight down, because you'll need adequate layup ( top n bottom) for the abuse (and potential water fill-up) that a cockpit must withstand.
Gasket material and design is very inportant. A friend of mine has a Willard 9 Ton, and the whole surface of the cockpit sole can be unbolted - one large pan head ss bolt on each corner - and removed.
I believe that Butyl Tape would be good. You'll want to be able to remove it once in a while, so the stronger adhesive sealants would be a bad choice.
This still does not address the issue of an easy-to-remove smaller access hatch.
There are plastic versions, but I am unsure as to whether they are strong enough. The cast aluminum ones are wonderful, but you'd need to see if there's room, as they stand proud a little bit.
It's a very worthy goal, but some planning and designing will have to come first.