Have you checked out the
SportRack a21030 cargo basket. This is a multi-purpose basket that attaches directly to your factory rack crossbars and is a cheaper option than the admittedly high priced options from Yakima Racks and Thule Racks. It will hold your duffel bags, luggage etc very nicely. The optional
basket Stetch Net to secure the bags and cargo gear in place is a nice accessory. Also their is a
SportRack basket extension available if you want to increase the cargo capacity of the basket.