I just purchased the BMW tail bag and have been very happy with the fit and function. Once you pull out the strap loops that are already attached to the underside of the seat, it takes all of 15 seconds to put on or take off. It has in internal rain cover and several exterior zip pockets. It also matches the saddle bags if that's important to you.
The part number is # 71 60 7 694 117, $167.00
I tried to find a link to the bag at a number of BMW sites but no luck. I took a photo of mine if that helps.