I'm currently part of a design team as a mechanical engineering student who is designing a rear rack mounted backpack rack which would attach to a standard rack (ie topeak explorer) and would either rest in a parallel or perpendicular position. In the vertical position, it would allow you to attach your backpack. When not in use, it would fold down, and merely rest on top of your rack. We are designing it to hold 25+ lbs. It's mostly about the design of the clamps which attach the two racks. I'll keep this post updated with our results.