I just bought a medium Banjo Bros commtuer backpack. It has plenty of room for a pair of shoes, pants, shirt, socks, and undershirt (I have a locker for shower supplies but It has room leftover after I put that sutff in it).

It is not a messenger bag but an actual packpack. So far I like it (day 2), but I still need to test it in a downpour to see if it is actually waterproof as it claims. My only gripe is that it currently sits a little high, slightly obstructing my reward vision. However, I haven't fully adjusted it to where I want it yet. The cost was $80 shipped...you should be able to find many other reviews if you google it.