Theft-proofing a convertible with no steering lock

Start with forementioned ignition system break the ignition feed, and especially if you have an electric pump

But remember, what it all comes down to is, they show up with a trailer / wrecker and have the time to hook it up. This doesn't take long if it's halfway accessable

So what? do you do?

I would investigate cell based tracking. If you have a "family plan" cell group, get an extra phone, and investigate how to track it. Figure out a way to hide the phone in the car, up above the glove box, etc