It sounds like you are just trying to secure it to the rack to prevent it from sliding off. If that's the case, you could just pop some grommets in there and use carabiner style clips to hold it on. I would say one in the front and one the rear should be plenty.

The other issue that you might face, depending on how you ride is the bag getting caught up in the rear spokes. The corrugated plastic or sheet metal would help you with that by stiffening the bag so I would be less likely to swing over into the back wheel.