My brother had given me a set of army green saddle bags some years ago that I've put on a couple bikes only to remember that without a solid backing of some sort inside will simply flap around and get caught up in the spokes. The problem worsened when weight was placed inside because it inevitably pushed the pannier bag inward toward the wheel.
To create the inner form to give the pannier some shape I used a couple pieces of coroplast. Coroplast is the material all those ugly yard signs are made out of such as signs advertising who to vote for in the next election, or that a place is for rent, or that the house is being re-sided by such-and-such a company. I wait until elections are over and then go around collecting signs that will inevitably end up in the trash.
I sized, scored, and folded the coroplast into the shape of a half-box, taped the bottom and sides together using clear packing tape, and then placed into the saddle bag. The whole process took a matter of minutes and was very easy.
I employed the use of any bike hackers favorite weapon - the zip tie - to attach the saddle bags over my bicycle rack. I also employed the use of a twist tie to hold the bottom of the bags to the vertical elements of the rack.
I've put a few dozen miles on the bicycle with these panniers and have already encountered some issues to which I estimate I will update and upgrade the design. I envision this hack being far superior if the coroplast sign were either sewn or zip-tied through the fabric of the saddle bag and then attached to the rack.