An immobiliser for my car. Had trouble finding devices that would cover the specific attack vectors my car would be susceptible to. Checked my insurance, no specific clauses around immobilisers, check relevant road laws, no issues there.
I have a fairly novel approach to operating it, and in the case of one time theft prevention security through obscurity is actually a great approach. The assailant only has a short time to pull the car apart and solve the puzzle, couple that with genuine security techniques, a physical aspect, and it should be pretty foolproof.
It can still be towed away, etc, not much to be done there except brute force physical blocks. Most cars get stolen here to do crime in that night so it's not as common.
I’ve seen implementations which need a magnet pressed to the door plastic in a specific place. Security through obscurity as you say but clever anyway - you basically need both a magnet on hand and know where to hold it.
Or time to pull the car apart
That's a really cool idea. When I was a kid my dad added an immobiliser to our car (flashing LED etc) with a hidden switch. It would have potentially taken quite some time to find if you didn't know where it was.
Adding a puzzle is brilliant and I would love to read a blog about this. Post it here on HN ;)