While I'm waiting two years (bloody neighbours objecting) for a council permit to build another garage this is how I try to look after the 240. So far there has not been a problem as long as the wind is not blowing too hard when putting the cover on.
make sure that the car is polished, clean, dry and all dust is off before putting a cover on.place a soft double bed sheet over the car with a honeycomb foam sheet over the roof (to allow breathing gap and provide hail resistance) then put the cover on and tie it down tightly to stop any rubbing in windy weather.
I recon that it works nearly as good as a car covered in a garage but is a pain in the neck to do every time I use the car.
The car without the cover.
The car with the cover on.
Laurie