Check out a Topeak rack, the one with the channel on it for Topeak rack bags. REI sells it for $26. If you use a Topeak bag with the quick release rails on the bottom, your bag just slides on. YOu slide it off by hitting a quick release on the front, then sliding it off the rails. It will also work with your regular panniers.
http://www.rei.com/product/47611606.htm?
http://www.rei.com/online/store/Prod...000&langId=-1&
http://www.rei.com/product/47802675.htm?
http://www.rei.com/product/47802673.htm?