Ortlieb sells most of the hardware to fix a rubber bag to attach to a rack as spare parts. Alternatively buy a used Ortlieb pannier and use the parts as a donor

Ortlieb sells most of the hardware to fix a rubber bag to attach to a rack as spare parts. Alternatively buy a used Ortlieb pannier and use the parts as a donor. The "trick" is having a stiff "plate" inside the bag for the mating hardware and bag to all attach to. You need to be very focused on heel clearance with a soft bag, ideally clinching straps pulls the bag corner nearest your heel taut. Also be careful of the location of the shoulder straps when the bag is mounted on the bike, I had the lugnut on a dublin bus inching forward in traffic pull off a strap. They need to stay out of your spokes but also not hang out to catch on passing vehicles either. Or just buy an ortlieb with the backpack kit.