The ShortStak Upgrade Kit is a stackable box that replaces the lid of a new or existing BlackPak Pro, giving you a second layer of storage for quick access go-to items. Its construction is virtually the same as the BlackPak Pro, however, it is only about 40% of the original height. The bottom edge of the ShortStak side panels is angled to match the geometry of the BlackPak opening, giving you the same water-shedding fit as the original lid.