I would suggest buying a Zeitgeist bag. They're built by Swift out of Seattle. I have the large one and it fits the few items I take to work with me. I do have a locker at work to store extra items. In the 11 litre bag I can fit my lunch, change of socks, work shirt, xtra tube. It has a two pockets located on each side which I keep a tool kit in one, lights in another. Check it out. It doesn't affect the way I ride either.