IF you want to add a combing to a flat deck there is another method.. you can make a rebated double coving to which to hatch slides into.. i just recently finished my latest sail boat.. which has that coving. although the coving stands above the deck by 5mm in this case there is no reason why the coving can't be achieved to give you a flat deck, it will however cost you internal depth space, and may not be appropriate if you have limited internal height.
There is no ''set rule'' on coving heights, all that's required is a tight fit and ther ability to keep water out, even the highest covings can lead to the loss of a wheel house or hatch if its a loose fit! The next few images show my new yachts and the coving depth is only 4mm and the boat has been drowned with no water ingress even after a 2 hour choppy race, the lid is a tight fit but can easily be removed with finger nail. Secured down with stainless steel screws which seal the securing holes.
Other methods are shallow covings, with high strength magnets to keep the hatch down , or the use of a pair of hooks one inside the boat, the other to the superstructure, and linked w a rubber band.