How large is this backpack (when compressed by bungee cords)? It sounds like the pack is wider than the rack will securely hold without the pack "rolling" off sideways. If you made a slightly curved cradle out of something attached to the rack, you could widen this support and probably make it harder for the bag to slide sideways off of the rack. Copper tubing or some kind of heavy guage wire mesh might work. I'm envisioning making something that runs the length of the rack, is about 8-12 inches across, and curves upwards as if shaped to securely hold a bucket or drum.