I have seen it suggested many times. Amazon and Walmart have them for $40 (
Allen Sports Deluxe Trunk Mount 2-Bike Carrier : Amazon.com : Sports & Outdoors), but would the rack itself be as useful as the straps? You can purchase replacement ones for $4 from
Replacement Upper Strap for Yakima Quickback 2 and 3 Bike Carriers Yakima Accessories and Parts Y90214, although those come without buckles, so it might be simpler to purchase the full rack.