Peter,
I bought a roll of hatch seal from Ian Porter a couple of years ago. It cost about £32 per roll, long enough with some spare to do both hatches (MK1) is black tape with a very strong sticky backing and is 7mm thick. It’s very easy to fit, you just need to cut a smallish wedge as you turn the corners…absolutely perfect. I wouldn’t mess around wasting money on anything else. I always remove the hatches after sailing, and the gaskets look as good as new. P.S…I am not on commission!