I was aboard a boat yesterday that had a bar that fitted into a keyhole plate on the back of each washboard ( inside ) and that was secured by a mushroom threaded fitting that dropped through the sliding hatch. All looked very secure and I'm damned if I can remember the name of it but it looked expensive, complicated and left a 10mm hole in the flat surface of the hatch to drip when not installed.
What about a garage door type arrangement? a handle on both sides and 2 bars that slide out to engage the hatch sides? Unlocked it would make a good hatch safety as well.