Dead Bolt
I used an old-style dead bolt like the one in the link. The price is right and it allows you to lock the hatch from the outside or the inside. I mounted the big part with the key on the top hatch board, on the inside, with the key side out and the bolt facing up. I screwed and glued a piece of teak block to the sliding hatch just the right size to mount the other piece so it would match up with the bolt. It looks good and is very secure and I really like that I can lock it from the inside.