Any of the boxes made by Thule and Yakima will fit the rack. Boxes by these companies are designed to fit the factory racks and the specialty racks. If you do a search online for these two companies you will be able to find specs and prices. If you want to go a cheaper but less sporty route I have a feeling that the Sears made boxes will work as well. I have personal experience with Thule products and would recommend them to anyone. I have sold hundreds of Thule components and everyone loves them. I know a good deal about Yakima so I would recommend you look into that company as well. Both companies produce similar products but everybody has their preferences. Happy hunting.
Matt