Attached to the ends of a tubular louver (1) with a triangular profile are corresponding stems (5), made of plastic as the louver (1) itself, having ribs (7) in the sense of their generatrices for a perfect fit, which ribs (5) emerge as a single piece from corresponding hollow end support plates (6), also made of plastic, in whose intermediate flat chamber are housed corresponding metal sheets (9) that are attached by harpoon-shaped recesses (11) to openings (12) of the plates (6) and are provided with an orthogonally bent lip (10) meant to facilitate its attachment to the base plate of the diffuser.