d 2 about 1 u00bd inches high welded to top plate 21 and shorter second transverse parallel ribs 34 of a third dimension d 3 about u215d inch high welded between the ribs 23 25 27 33 and to the top plate 21 for strength the ribs 27 and 30 include l shaped bottom edge flanges 35 and 36 that extend toward each other to define the bottom of a horizontal track for receiving a flat dampener panel 38 the bottom surfaces of the ribs 28 and 29 form a top of the horizontal track for sliding engagement with the dampener panel 38 the ribs 30 and 33 include edge flanges similar to flanges 35 and 36 to define a second horizontal track for dampener panels 38 the illustrated dampener panels 38 are flat sheet metal components shaped to temporarily flex to slip between the edge flanges for assembly however it is contemplated that the dampener panels 38 can be made of other materials such as plastic and other non metal materials the sheet metal is resilient such that it springs back to an original flat shape i e it u201c unflexes u201d such that its edges engage the track for adjustable movement along the track between different overlapping positions this allows for very low cost assembly allows the use of low cost components and allows for adjustable controlled air flow the illustrated dampener panels 38 include a single short slot allowing top.