goto a pet store and buy a product called "bitter apple", it comes in a spray and is made to keep cats off of things (furniture, x-mas trees......porsches).
but some cats really like it, so do a towel first and see what the cat does.....if you don't introduce it to him he'll probably hate it (i.e.....leave it on the floor for him to discover himself)
simply lighty spray your car cover (don't need to saturate it) and the cat should stay off.....and it's "humane".
this is what my gf michelle said to do
