I picked up a bag at a surplus store the other day that makes a perfect seat bag. Big, tucks nicely under the seat, lots of external loops for hanging my blinky lights and stowing sunglasses, and my cable lock fits perfectly in the external pocket. I'll have to get a photo of it and see if I can find an online source.

Added: Here's the only photo I have of it so far, from a lousy phonecam pic, but you can see how big it is.


Added more: Found it with a google image search and some well chosen keywords. Interestingly it is a water bottle carrier and the Amazon listing shows it with a Camelbak hose coming out of it. No sign of a reservoir to fit into it, however.