Thanks for the suggestions guys
Plenty of food for thought there.
For other reasons I really need them to be completely flush with the deck when not in use.
Unfortunately Polwarts Monitor marine items look particularly good but I don't know how I'd mount them, and I bet they are a bit pricey.
I don't really want to go adding hatches everywhere to gain access to the underside of the deck, and it doesn't matter if the threaded holes are left open when not in use. The area underneath has the fuel tank in it and has large drains back into the well at the transom. Any water in the well backs up into the space anyway, so I often remove the small hatch over the sender and flush it with fresh water.
I'm thinking more about using 3inch discs with 4 screws in each point along with epoxy. With 4 points the engine would have 16 screws and a large area of epoxy holding it down which I'd have thought would be enough.
Need to do some more thinking.