If somebody really wants to get into your boat, they will get into your boat, no matter how many locks and alarms you have. My goal was to provide enough security to keep honest people honest. It took a lot of head scratching but this is what I came up with.
It’s an L shaped piece of aluminum that slots through a notch in the hatch rail. It’s thick enough to keep the hatch from sliding forward, and the lock prevents it from being removed. When inserted one way it allows the hatch to be left open that crucial inch for ventilation. When flipped over it locks the hatch completely closed.
I made slots on both hatch rails to make locking from either side possible and I made two of the aluminum Ls in case I lost one or wanted to double lock.