I have some shopping panniers from REI that hold a good load and fold flat against the rack when not in use. They are open on the top, however. I don't think it is a current product anymore, but Performance/Nashbar sells a very similar one. I just checked Nashbar's offering, and it comes with a rain cover. $20 a bag is less than I paid for my REI version and I got a good clearance on mine.