I got the idea from an ancient Reddit thread ages ago, figured I'd revive the super helpful advice with a picture!
Didn't want to bother getting a pannier, but my shoulders got kinda tired on the commute, so here's the solution with the rear rack. Just wrap the straps around the seat post, fold backpack down as best you can, then just wrap it up with a bungee cord. Works great!
by JenJub