I have a Rainguard Lightweight Waterproof Adventure Motorcycle Cover in 'Large'which I bought earlier this year for £15.95 on Amazon. It fits a GS LC with full vario luggage quite comfortably and packs down to an easy to stow size. Mine lives in my top box so it's always on hand. The only criticism I have is that it looks very expensive, but it covers the bike completely so passing Herberts can't tell what's under it.
Just to add: I've often covered the bike immediately after riding with no ill effects from the hot engine or exhaust. I don't use it at home, so it doesn't have a particularly hard life.