I'm very happy with the bag I got from USA as shown here -
http://motorcycle-accessories.cruise...59&model_id=99
Used it for a Triumph Owner's Rally last week-end and it was more than big enough, even without opening the expansion zippers. Once I'd worked out the best way to fit it to my sissy-bar, it went on and came off with just 5 clicks.
Although I didn't use the feature, you can relocate two straps to turn it into a back-pack. It won't be the most comfortable, but for short periods it's probably OK.