I made a square drain channel mold plug, with PVC pipe, sprayed it with pam then laid wet mat over it, Once set up it required some trimming before glassing it to the underside of my deck
While the deck was upside down out of the boat I glassed the drain channel to the underside of the deck with 3 layers of mat. Then came the fun part making the channel the right level to support the hatch. Once level I laid mat from the channel rolling it up onto the deck. After much sanding, fairing and fitting of the channels and hatches I used plastic molding over the edge of the channel that supports the hatch. Non of the hatches are identical but they are close enough only I see the imperfections. I then bought flush mount hinges and good compression latches so they are tight when down and do not rattle when running.
I'm sure there is a better way to do it but that is what I came up with using my limited fiberglass skills.
I made all my hatches the same size, just large enough to fit a 5gal bucket or milk crate in.
I have seen hatches for sale where they pop in from the top side then are glassed down. Let me know when you get close on a mold as I would like to see what you came up with.
Capt matt
