Middle Atlantic RIB-1-MRK-42 The Middle Atlantic RIB-1-MRK-42 is a 2.5 inches (63 mm) riser base that can accommodate a single MRK series 42 inches (1067 mm) deep rack. Use this riser base for field rough-in prior to arrival of racks and enclosures. For advanced cable management, it has laser knockouts on its both sides for passing the cables. This riser base is phosphate pre-treated with black powder coat finish. Its structure uses 11-gauge steel with 0.12 inches (3.2 mm) thick fully welded construction.