if you don't mind spending a little money for quality, jandd makes some great seat bags in lots of sizes (i've got a few, from the very basic tool bag to the mountain wedge III which can carry pretty much everything you need for a month on the road plus your pet cat comfortably). Some of the smaller ones have a quick release mechanism so that you can detach the bag and take it in with you, or leave it at home for short trips w/o needing to remove all the hardware.