I fitted one of these nifty Givi "Tankloc" bags. Appealed to me mostly because it doesn't contact the paintwork. Can be removed and replaced in about 1 sec! Mechanism is simple design and looks sturdy and should give long life.(http://farm8.staticflickr.com/7012/6689246711_f602406e35_z.jpg)
And this will be the photo that should accompanied the previous post.(http://farm8.staticflickr.com/7019/6689257793_aa489f8669_z.jpg)