For the most part, a car cover, "is" a good idea, or should I say, a starting point at least, although having the added aggrevation of putting it on and taking it off every day, would certainly get old... QUICK, IMHO. With a car cover, my 1st and foremost
concern would be, the cat [literally] "kneeding" on the cover, preparing itself a spot to lay on

, effectively rendering the cover "useless", by tearing to shreds!!!

Not to mention, that the cat's claws, would certainly be in use, possibly causing some REAL paint damage!!
I'd have to agree with (wylcat). The pellet gun, sounds like your best option. I recommend that you look for a pellet gun, which you can "pump-up". The more you pump it up, the faster the velocity of the pellet/bb, that way, you can control the amount of velocity of the pellet/bb and be... somewhat.. humane about it. Still though, you definately need make it fast enough, to make the cat
learn a VERY QUICK lesson. This approach "does" work. I'm sure you've heard the old saying...
"once bitten, twice shy".

You don't "necessarily" have to shoot at the cat, while it's on your truck. If it were me, all I'd wait for, is the chance just to "see" the darn thing, on
my side of the property line!

Needless to say, that's when... I'd fire its little a$$ up!
Just my .02 worth...