Have been thinking more about this. K.I.S.S. Drill a hole through the frame and dropboard that goes from the outside to the inside of the cabin
. Get two snugly-fitting clevis-type pins or rods that will slide into the hole to lock it, with lanyards to keep them conveniently nearby, one inside and one outside. Pushing the pin in from one side will lock the board(s) in place. The pin on the other side can be used to push the first one out, and then be pulled out as well, so the board(s) will be unlocked. The pins stay either inside or outside, and the boards can be locked or opened from either side, or left unlocked with the pins hanging. This type of pin https://hardware.schaefermarine.com/...oducts_id=3810
might not be long enough, but will give the idea. A small block might be installed to drill the hole through if the frame of the hatchway called for too long a hole.