I came up with a simple way ,for me, of making sure my hatch seal works.My construction methods for making the hatch lip mean a poor coresponding contour with the hatch rim packer and wide faced gasket so inevitably providing a leak path.My available gasket material needs some force to compress it if over a larger area and I want the flush hatch/deck surface.My catch up came when I put the two together by making an inner upstanding thin lip ring out of 1/16 ply ,with outside fillet and glass.
This alows for deep gasket penatration with diminshed force to do so.The whole ring gets to kiss the foam ,some parts deeper than others ,due to my poor contours.It is now too good so a little breather hole in a seperate inspection cover helps with the effect of those gas equations.

I Hope this helps.It shows, from near to far,the up side down hatch with shock cord hook, foam ring gasket,2 x 1/8th ply rim packer.Then, on the deck,the sharp upstanding 1/6th ply ring [to penatrate the foam ring].Behind the sharp ring and out of sight is the fillet onto the deck lip surface.Then, 2 x 1/8th ply deck lip top surface , 2 x 1/8th under deck lip packer and the deck shell.It is a big hatch so it "needs" considerable stiffness which the generous ply laminates provide.ignor the back ground, if you can.
cheers from Paul .nz