I bungee my backpack to the rack and have not had a problem. I use 2 bungee's in a crisscross pattern. The Rack I have is the Axiom Odyssee.

Looking at the axiom website I would look at the streamliner road. It says it allows for more heel clearance. On the bottom they also sell luggage straps that should work.

If you are worried about rain, I cover my backpack in plastic garbage bags. You can also purchase rain covers for backpacks as well.

Panniers carry the load lower on the bike. This helps with stability. Also Panniers can give the illusion of making you look wider than you are making cars give you a wider clearance.