StarTech.com 1U Vertical 1.8 x 3.9in Server Rack Cable Management D-Ring Hook w/ Flexible Opening - Network Rack-Mount Cord Organizer Ring (CMHOOK1UN) - Cable management ring - black - 1U - for P/N: RACK-18U-20-WALL-OA, RACK-21U-20-WALL-OA, RACK-24U-20-WALL-OA, RK3236BKF, RKQMCAB12V2