Using 2 car covers....
Urine does not work - even tried badger piss you can buy online, cats smell it and get inquisitive.
Travis... on a serious note... car covers are made to breathe to a certain extent so as to not trap in moisture. Using two should be fine for a short period of time. You can get cat repellant in a spray bottle and can get powder for the ground. Spray the areas of your car cover then make a barrier around it.
If that doesn't work well then come over and get this:
If that doesn't work well then come over and get this:





