If you try what Saddlesores suggests, a J hook, not sure how you would attach it to the pannier. My panniers are over a decade old, they use a M5 bolt to hold the hooks to the pannier rail. If yours use the same or similar hardware, an M5 bolt might be sufficient.

A huge number of panniers that had J hooks have been sold that only used bungee cords to pull the pannier downwards to keep the pannier from jumping upwards when you hit a bump, if the bungee can hold it down well enough, the pannier stays on the rack. But, the J hooks need to be strong enough to hold the weight in the pannier when you hit bumps and also the extra strength needed to stretch the bungee cord.