You will want to stick with one brand throughout the entire roof rack kit; various brands of roof racks are not made to be compatible and interchangeable with each other. The most economical roof rack for your 2008 Scion xB is Inno Square Crossbars - Steel - Black - 58" Long - Qty 2 # INB147 with Feet # INSUT and Fit Kit # INK300. This has a weight capacity of 120lbs. view full answer...