What sort of seatbag do you use? I've been looking around for one, because I feel really vulnerable riding without a flat kit, but I can't seem to find one that fits my needs. Ideally, I'd like one that is easily (and quickly) attachable/detachable, so I can take it with me when I lock up.

'Course, I gotta get all the other stuff that goes inside, too: multi-tool, 15mm wrench, extra tube, CO2, etc.