Abstract: A substantially airtight ducting enclosure is provided for a heating, ventilation, and/or air conditioning system. The enclosure, which may be a register box for return air or supply air, has a body formed from at least one planar sheet, the body having an inlet and an outlet. At least one of the inlet and the outlet is formed by folding corners of the planar sheet to form planar sides that lie in intersecting planes. The enclosure is then mounted to a surface of an interior area of a structure, and connected to ducts for communicating air between the interior area and a central air-handler unit.