I use a Rivendell Bilbo Baggins seat bag, which is pretty large but can also haul a bunch. They make something similar these days, just calling it something else.

It's nice as it can carry all the extra crap I don't want in a R jersy pocket and/or when I'm simply wearing a tee-shirt, including all the spare tubes, tools, CO2, etc...

The canvas isn't waterproof, but a zip-loc baggie is better at keeping dry the important stuff. Canvas and leather straps are much more durable then nylon stuff with a zipper.

SB