C-Box HVAC Plenum Ceiling Box 12 x 12 x 8 Round-Connects to Vent Register Diffuser Our C-Boxes are built with a 1/2&quot; flange all the way around the square portion which can be used for attachment or folded over. We also have a full line of ceiling registers and hvac fittings. The best way to regulate air flow is to install a manual volume damper in the duct run. Connects a 8" Round Duct to a 12 x 12 Diffuser or Register Vent. Call us with your hole size and duct size so we can help (866) 309-9899