I use one from Aerostich. If they have more than one, I don't know which model it is since I've had it long enough to forget which one I ordered.
It's very light, stuffs up into its own attached "stuffsack" which is about 5" thick by 9" long with the cover inside. It's a really neat little package. Takes up next to no room in the topcase. I can put it on the bike in less than a minute and take it off and have it "stuffed" in about the same amount of time. It's also waterproof (tested and verified by me). I wouldn't use it for longterm outdoor storage, but for on the road use, it's perfect.
I agree with the comment about never using one of these when trailering. That would definitely be fun to watch (from a distance) as it went flapping down the highway but you'd never arrive at your destination still in possession of the cover.