Jenks
I'd suggest the only decent covers are those that need two panniers to store them in -so taking a cover while you're touring . . . . where the heck will you put it? And roll it up when it's wet?
I'm thinking you're after one of these for overnight 'camouflage', but in my experience you can normally get good secure parking at your accomodation.
If it's for rain proofing, when you wake up and it's raining - the bike's gonna get wet anyway riding down the road. If it rained overnight but is a dry morning - enjoy it!
The bike wouldn't care if it rained every day, because it's a WMB.
Or have I missed your point?
Cheers,
Neil.