As others have said, this won’t stop a determined thief. But I also fear predatory lawyers, who will sue me if some teenager steals and crashes my plane because I left it unlocked (‘attractive nucense’).
1. As noted above, a pin in the internal, inside door mechanism, can prevent the external button from moving in. Even simpler, I sanded a small piece of pine to wedge into the mechanism to keep the button from depressing. Painted it red, to remind me to remove before flight. After installing the pine, exit thru the baggage door.
2. Exiting thru the baggage door is a pain, so I also have two telescoping tubes that go over the external handle, keeping anyone from pushing the button. The tubes are locked together with a padlock. Just aluminum, won’t stop a real thief, but at least I made an effort.