@IanTurk Try Blutac. Roll a thin tube of Blutac and apply it around the periphery of the hatch which should made larger than the hole cut in the deck. This will "stick" the hatch to the deck and make it watertight. The hatch can be "levered" off as required. The blutac is then rerolled again into a tube and restuck to the periphery of the hatch and pushed down on to the deck again. Its easy inexpensive and lasts for a very longtime

@jumpugly Thanks - yes tubing, great idea. I have a stash of rare earth magnets, that I knew would come in handy one day!

@IanTurk
Hey there Ian!
I used small 6mm rare earth magnets on added cross members spanning the hole in the hull, and some rubber tubing (not vinyl) on the underside of the hatch.
Or the reverse, so the tubing rides slightly underneath and along the cut lines in the deck where the hatch was. It's a bit ticklish, but totally doable.
😉👍