The ceiling box is the best way to regulate airflow is to install a manual volume damper in the duct run, all the way around the square portion which can be used for attachment or folded over. Our galvanized duct boxes are made of high-quality galvanized sheet metal with crimped ends (snap-lock design) for easy fitting and secure connection. The c-box duct with drywall flange, round-connects to vent register diffuser, airflow conditioning ceiling registers.