You might not need a cargo box. Maybe you just need a place to stash some stuff that doesn't matter if it's exposed to the elements. "Like what?" you may ask. Well, waterproof duffel bags for one. A mesh bag of old neoprene paddle gear that is a little, well, fragrant. Maybe your dog kennel (sans dog).
Plus there is the Safari Effect, where the basket make everything look a little more Land Roveresque. Even a minivan.