The Marsee tail bag has a harnass which goes under the seat and then you zip the bag onto the harness. This sounds like a sweet setup. Anybody with the Marsee tail bag. I guess one can use the Rocket Pocket or Mr. High Pockets tail bag with this base. Downside it's expensive. Anyplace to buy them cheap?
Before you make a purchase you should really consider the Ventura tailbag system. It has a rack that neatly and quickly mounts into the holes for the rear passenger grab handles, and it will hold up to 80 litres of gear when both bags are used. It can even be used when riding two up.
Sorry i don't have a link for you, but if you do a search (Google) under Ventura Tailbag you should find it.