Topeak Pro Pack-Small
http://www.rei.com/product/751362/topeak-pro-pack-small
This is relativly small but it is durable and I am very happy with it. It is rigid and has never once fallen off. The clip attaches to the rails of the saddle. Screw it on with an allen wrench and the "bag" snaps right on. The larger models have a velcro band to go around the seatpost. You may want to bring your saddle if this product interests you to make sure the attachements will fit. I have a Selle San Marco saddle and it barely fit the rails. If I had to rate this product I would give it a 5 star out of 5 stars and have had it for over 2 years now.