Hi,
We have a few neighbourhood cats and as my car sleeps outside I've experienced this problem. Both my BMW convertible and Volvo have scratch marks on them where cats have slid on the polished surface.
When I bopught the Boxter I was determined that this would not happen so I bought a car cover. I just throw this over the car at night and it doesn't matter if the cats sleep on it. I expect potentially this could be worse with the Boxter as the front and back are very curved offering little purchase for the cat.
I've heard of cat repellant and chilli pepper on the roof but fear that this might damage the top. Bonkers Bob set explosives in his car and detonated when the cat jumped on board, it worked but at a price
