I've got one of these for the M3:
http://www.hamiltonclassic.co.uk/acatalog/OutdoorCoverCarBreathableSoft.html" onclick="window.open(this.href);return false;
Not that it's outside, but it came with the car. Excellent quality and would be great outdoors if needed. Great as an indoor dust cover.
Might be an idea to read this on general cover advice:
http://www.hamiltonclassic.co.uk/acatalog/Car_Cover_Advise.html" onclick="window.open(this.href);return false;