Locking gas cap

[fredonthehill]

Good afternoon all.
So I filled up the other day, then went for a cruise with the family the next morning I noticed that the gas gauge was down two full bars when I started things up, so I think I am the victim of gas theft...which doesn't surprise me given that gas is 5.99 here and two big winches were pulled off a sailboat at 4 AM a few slips over.
Before I get accused of getting different readings because of the boat's position being a determining factor in the gas gauge reading, I will say that when the gauge read full at the pump, I was stopped at dead calm....same thing when I read the gauge the next day.
I went on Amazon and saw a bunch of different sizes, etc. Can someone provide some clarity on a specific cap that will fit the boat? I would prefer to just buy something, screw it on and lock it, as to taking off the original flange, disconnecting the hose, etc.
I own a 2022 Parker SL SC.
Thanks!

 

[Roll Tide]

Most of the locking caps I've seen come as a whole assembly.

You can try something like this that prevents siphoning from the fill:

http://www.vetusmarine.com/store/p/387-Fuel-Theft-Security-Device.aspx

 

[Navigator]

I would think a locking gas cap wouldn't make it one season. As soon as saltwater got into lock mechanism and did its thing either jamming it up or corroding it to a solid mass you would be done.