Another vote for timbuk2. I have a large one that I currently use for my commute and used to have a XS one. I'm not quite sure what happened to the small one, but I suspect my wife accidentally gave it away during one of her cleaning binges. It was probably a bit bigger than what you are looking for, but it was perfect for just a few items. I would carry pants and a shirt in it. Anyhow...nice, comfortable bags that seem to last forever.