of fluid flow and of which a side is shown on fig3 c and presents a rectangular shape comprising a first end 88 extending in a first flat end 60 of a long crushed prolonged surface 80 of a width of 1 u00bd inch and a first angular crushing 82 at 45 u00b0 relatively to the top 24 a second end 89 extends in a second flat end 62 and comprises a second angular crushing 86 at 45 u00b0 relatively to the opened side 30 the second flat end 62 is crushed on a width of u00bd inch and can comprise a small prolongated surface 84 that overhangs the second angular crushing 86 0047 fig3 c illustrates the fourth step of construction namely to assemble the rectangular air duct 20 when the sandwich sheet 22 is folded to form the air duct 20 the first angular crushing 82 and the second angular crushing 86 join to make a corner of the air duct the first flat end 60 located at the first end 88 is to be stuck to the outside of the opened side 30 and the second flat end 62 located at a second end 89 is to be stuck on the second crushing end 86 and on the small prolongated surface 84 on bottom 24 to form the air duct 20 0049 fig3 d illustrates a detail of the location of the first flat end 60 once assembled and held by the adhesive band 40 the fifth step is that of fitting the rectangular adaptor 42 into the downstream end 36 and to fix the c reinforcement 48 fig1 inside the air duct 20.