Disappearing car toppers

we’ve had a few car toppers go missing over the two years we’ve been open. We are on a busy block with street side parking. When we had the last problems we got drivers to take the car toppers off when outside the shop but it either meant drivers didn’t put them back on or it was hassle taking them on/off.

Has anyone found a way to secure these? I was thinking of a seat bolt type material which we secure to the topper then trap in the passenger door.

Anyone had similar problems/solution?

I have not tried them so I can’t say how well they work but here is the link.
http://www.carsigns.com/SignLock.asp

I replaced all my signs this spring with ones from HTH with the above mentioned sign lock. It works well and have since only lost 1 of my 14 signs. I’m sure the one is sitting in an employee or former employees garage. You can easily fabricate your own version of a “sign lock” if you want.