Google Nida Core, Divinycell, Core Cell.
All of these require you to construct the hatch by glassing the proper thickness to both sides.
The hard part is glassing the edge as GRP does not like 90deg corners.
If the hatch is flat , there is sheet GRP of good thickness , say 1/2 inch that could be fitted like ply wood.
If the hatch is not too large simply fitting a commercial aluminum deck hatch works very well,
We have 6 of them , none leak .
Baier Marine Hatches
There are many other brands.