I have only used marine ply 3mm laminates.two for the lip and two for the lip to deck packer which also is cut for the hatch rim.I put the gasket on the hatch ,just inside the rim.I also make a ring upstanding to make sure that this penetrates the gasket material all the way round.The gasket can be between 4 to 5mm thick.I can't rely on my ply lip laminating process being exactly true to the deck contour hence the penetration ring which is a ring of 1.5mm ply glassed on the inside of the lip.this gives a very reliable seal,most of the time...
I hope this helps.