With regards to Hylomar gasket sealant. Have any of you used it?
- have on many engines in the 70'a & 80's. I can tell you its a brilliant sealant, but it is not re-sealable! Once you remove your plate, engine cover or whatever, it will need to be scraped off and re applied, it only semi cures in an engine environment so making it relatively easy to remove compared to the silicone variety's.
In short I wouldn't want or advise to use as a hatch sealant, it will cause more trouble than it is worth.
What you could do is lay a bead of kitchen/bathroom silicone around your opening, apply some veg oil to you hatch cover or wrap in cling film. Then lightly lay into place just enough to squash the sealant. Leave it for a day to set. Then remove the hatch, leave another couple of days to fully cure and you have a good watertight gasket that you can fasten your hatch down to with a couple of screws/bolts or whatever you are using.