I was walking from the bus stop through the kamong to Ong's marina on the Straits of Johor side of Singapore when I saw a bloke walking towards me with a fisherman's anchor over his shoulder.
Long story short, it was mine.
Talking to the other yotties and found that there was a fair amount of small time pilfering going on.
I made up a simple alarm and when others heard of it, I was in business.
Didn't end up as Elon, but had 10 satisfied customers and many used these for years sailing through SE Asia.
The alarm will activate a motorcycle horn when the sliding hatch is moved 30 mm or so.
This will not prevent damage to the hatch or lock but will send any thieves running and save your gear below.
The timer relay can be set to limit the sounding time, saving the battery from running down. 3 or 4 minutes is plenty.
I placed the relay horn and indicator relay in a tupperware box and had all wires plugged in. Easy to remove and stow when not needed.
Part 4 is a micro switch that is fixed to the very forward end of the inside of the hatch rails. There is velcro on the switch and velcro on the rail so it is easily removed.
Part 2 Is the isolator /activator switch. Most were fitted in cockpit lockers.
Part 6 is an optional car indicator relay. This gives the horn a pulsing sound.
All parts a readily available and cheap.

You can also consider one of those 80Dba Murata self driven piezo alarms, they are cheap as well. Same goes for the Alarm panel horns which are waterproof and UV resistant. Many of these horns also have a flashing LED alarm light or even a neon triggered strobe which is a real attention getter. These days they are mostly LED.

I would also put a 1n4004 diode or bigger in series with 12 volt positive, its safety measure as well as preventing transient back emf on the relay.