Middle Atlantic RIB-3-MRK-42 The Middle Atlantic RIB-3-MRK-42 3-Bay Steel Riser Base is a 2.5" (63 mm) riser base that can accommodate three MRK series 42 inches (1067 mm) deep racks. This riser base allows for field rough-in prior to arrival of racks and enclosures. To facillitate cable management between racks, this riser has laser knockouts on both sides. With a lifetime warranty, this base is constructed using 11-gauge steel pre-treated with a black powder-coat finish