The Carradice Super C Maxi Stuff Sack is a great complement for racktop cargo hauling of softer, packable items. At a cavernous 30L volume, it can easily swallow up bulky items. Works great when wedged between two panniers and fastened down with bungees or straps(not included). Made of super tough cotton duck material, this bag will take pretty much take anything that comes its way.
A roll top closure allow for resizing the bag when not at full capacity as well as keeping it highly water resistant.
While the bag’s fabric is waterproof, the bag is not watertight. If you’re riding in inclement weather, it’s a good idea to make sure that moisture sensitive items like electronics get some kind of extra measure of protection.