A couple of comments and thoughts.
Gas theft is problematic with boats - both on the hard and when moored. Boats are low hanging fruit in both settings.
Without being able to see your current fill cap, I can't really say what options you have for a locking cap or something similar.
You could switch to using removable gas tanks instead of the integral tank and keep them stored safely until needed. With a 100 litre tank, a medium sized 4 stroke outboard engine, and a relatively light aluminum boat, your fuel burn isn't going to be massive. As you said, 20 litres was enough to save the day.
One thing that I regularly do is keep my tanks completely topped off before launching. A partially filled fuel tank is going to "inhale" some humidity and over time water will start to accumulate. A filled tank doesn't breathe. Fuel theft sucks, but if you went to fill your tanks before hitting the water, you'd have noticed it and filled up.
Food for thought...
Anyways, you could check out Attwood's website for gas cap ideas or options for your application. Good luck!