A Small Backpack
This needs to be small enough that when you wear it, it does not cover your ass AT ALL. It only needs to be big enough to hold one outfit, so the smaller the better.
Also, the backpack needs to have double-zippers on the main compartment. You know what I mean right? It's got two zippers that zip together in the middle. And, these zippers HAVE TO be the style where there's a little round hole in the handles. Hopefully that makes sense.