Bedsheets aren't usually big enough on their own so unless you're handy with a sewing machine you'll have work to do.

A double duvet ripped open lengthways works,

but the best thing I've discovered is from Ikea, you can buy unbleached cotton sheeting from there by the yard for pence.

No dyes or anything to upset your paintwork, and you can buy a single piece big enough to cover the car.

Mine now lives under one of them even during the summer.

It keeps the dust off between polishes.

Karl