Lock the car leave the keys in the drawer of the office and call the car in stolen when it gets in an accident.

Seriously I have heard of other airports that do not have a courtesy car for insurance reasons. The AOPA and EAA ar so into selling insurance. I wish they would come up with a plan to cover airport cars.

You'll probably have to swing for a commercial policy with a "non-owned" clause. I have several company vehicles that my employees drive and as long as they have permission and a valid DL anyone with permission can use them with full coverage....just have to have a primary insured.