While it is distressing to lose equipment in this manner, a good insurance policy would seem to be the way to take the pain out of it a little.
I have cover for guitars, when left unattended in a parked, and locked, vehicle. The only proviso is that the items must not be visible from outside said vehicle. So get a load cover for you estate car, and use it, or drive a van,
It's also no use covering your car with Folk Against Fascism, Musician's Union Says Keep Music Live, and/or various folk festival/club stickers. Then expecting people with nicking in mind, not to want to investigate the contents of your vehicle, covered or uncovered!