Theft-proofing a convertible with no steering lock

I have a piece of vinyl black tubing with two spark boots on it. I remove the wire between dizzy and coil, and replace with that. Looks like a wire, but is just a hollow tube. Then I pocket the coil wire and go about my day.

But with a convertible they sell alarms that will put up a "bubble" and if it senses any movement within that bubble, it will set off the alarm and disable the ignition. It is inteded for convertible cars, so if you have the top down and someone reaches into the cabin, it goes off.