Just remembered looking at motorbike paniers clipped to lockable mounts on the rack.
Givi is a good make.
Not cheap though
I have used this approach, I have a pair of 35litre Givi Monokey panniers and a rack left over from my motorcycling days. It's easy to mount the rack, the pannier locks on very securely, and they are of course completely waterproof.
The big disadvantage from my point of view is that they obscure rearward vision more than I like, which is why I switched to a Peli case. Just as waterproof, lower profile, but the tradeoff is a smaller volume. Fine for a couple of nights, though. Any more and I use a soft leather holdall behind the seats.
