If that's all you want to carry, try a Carradice saddlebag. It'll be lighter, cheaper, and more stable----plus it will outlast YOU.

http://www.wiggle.co.uk/carradice-ca...lap_Saddle_Bag

It works best on a Brooks saddle, which has bag loops. If you're not riding a Brooks, there are add-on bag loops.

http://www.thebikebiz.com/ViVa-Bag-L...-acc-via01.htm