The image above provides the APPROXIMATE measurements of the opening in the wide-mouth Kayak Foam block.
This is a PAIR of 16 inch long kayak foam blocks for extra wide crossbars. This wide-mouth foam block can be used on the wide Xterra factory crossbars or can be stretched wide enough to be used on TracRac crossbars. Each block measures approximately 16 inches long by 4.75 inches wide by 3.875 inches tall. Manufacturer's part number: (Qty 2: KFXSUV)