Abstract: An outlet for conditioned air has a plurality of interconnected modules establishing its side walls which support a closed bottom and by which the outlet is connected to a mounting frame. Each module has a supporting frame consisting of upper and lower side members and end members, each a length of an extrusion providing an outer wall and an inwardly disposed wall. Means are provided to interconnect the frames to each other vertically or laterally in the same plane or at right angles in a manner providing air tight seals.