I made a pair of panniers and tried out a few interesting ideas. They worked well for several years, but were ultimately kind of a pain to put on and take off my bike. One pannier had handles like a brief case, and the other had a shoulder strap like a messenger bag.
The panniers attatched at the top with two webbing loops which hooked over posts on the rack. The posts were 3D printed.
A strap at the bottom of the pannier pulled it down and stopped it from coming off the posts. This proved a very secure attatchment strategy, but the hook was cumbersome.