Well, I've seen at least one that looked good - Arkel's "The Bug" -
http://www.calhouncycle.com/productc...idproduct=1087
http://www.arkel-od.com/panniers/bac....asp?fl=&site=

But for a much cheaper and more versatile option, I would get some of those folding wire baskets that attach to the side of the rack, then put my regular backpack in those while I'm riding. Depends on your uses, I suppose.