This and this have worked just fine for me for 5 years and thousands of miles. I cut off that silly lip on the rear rack so my rack pack will slide up under my seat.

I am pleased with my Axiom non waterproof panniers with covers. Got the Seymore and LaSalle. LaSalle comes with covers. Use them a lot for their visibility factor. Obviously, garbage sacks will keep your stuff dry. Those used in trash compacters are tough and sized about right.