You know just thinking, I would put a Massive sign on your rear window & your drivers window when you leave your car. I think there would be more chance of the one in the rear window being noticed.
OR when you park your car with plenty of space, go find a trolley & put it next your car. Maybe people wont go to the effort to move the trolley to park there & just move on to somewhere else.

eta - or purchase some witches hats & put them along the side of your car LOL. People would be less likely to move those.