Un-garaged boxster?

[Perfectlap]

unless it's a car that will be parked outside and driven only once in a blue moon, outdoor car covers are not a good idea, particualrly with darker colored cars. Car covers should only be used when the car is 100% clean. If its dirty, the inside of the cover will get soiled and will act like sandpaper each time you put it on the car.
If you properly wax and seal the paint it will easily withstand the elements.
I'd recommend getting a hardtop if the car is going to an outdoor parked and daily driven car. Soft top covers (only covers the canvas) are also good for outdoor parking, check ebay.