C-Box HVAC Plenum Ceiling Box 6 x 6 x 6 Round-Connects to Vent Register Diffuser by SMK Connects a 6 Our C-Boxes are built with a 1/2 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 (866) 309-9899