Todd, on a North Bay I made a year and a half ago I cut out two circular hatches in the front and back. Using a crude pencil and string compass. The hatches were about two inches from the shear clamps. Oh and the deck had been glassed with 4oz. Using 3mm make a spacer that is about 1 1/8" around the hole and connecting to the sheer clamp. The spacer was made out of 4 pieces, looking kind of like a Y with a quarter curve following the hatch with another leg connecting to the sheer clamp. That way the hatch is stiffened with a sheer to sheer deck support tangental to the hatch hole. The 3mm doesn't distort the deck upon installation and provided barely enough recess to get the hatch almost flush with standard 3/8" weather stripping. The recess ring is made out of two semi circles sitting 1" on the spacer and extending in 1". When all is glued and cured the inner ring can be cut/sanded back to 3/4", the stepped recess and spacer are sanded to make an easy transition for a layer of glass to support the underside of the ring as well as connect to the deck. All this may be overkill if you can do it with 4mm. At least the butted seams for and aft of the the retainer need glass underneath. The hatch cut out will probably need a rib to get the curve back to match the deck, if the deck wasn't glassed then glass the under side of the hatch then attach rib when poxy is green/not tacky. My front hatch was bone dry. The rear hatch wasn't dry enough since it was flat so I'd try some other hatch tie down.