C-Box HVAC Plenum Ceiling Box 18 x 18 x 18 Round-Connects to Vent Register Diffuser Connects a 18" Round Duct to a 18 x 18 Diffuser or Register Vent. Our C-Boxes are built with a 1/2" flange all the way around the square portion which can be used for attachment or folded over. The best way to regulate air flow is to install a manual volume damper in the duct run. We also have a full line of ceiling registers and hvac fittings. Call us with your hole size and duct size so we can help (707) 999-7712