air-duct flange:

Plane flanges permit ducts to be joined with extremely low leakage.
The panels are joined in their width sections through the "H"-type
bayonet structure without using any gaskets. Generally they are made
of PVC in 4 meter lengths. Ideal for the creation of take-offs and
tap-ins and connection into the plenum chamber. Also available
are "U"type, "F"type, "H"type, and "Y"type aluminum flanges.