and 13 as follows first a longitudinal cut flush with the inner surface of the upper panel wall and running the full width of the panel is made to the desired longitudinal depth subsequently to or simultaneously with that cut a parallel cut is made flush with the inner surface of the lower panel wall finally a vertical cut is made transversely through the entire width of the panel at the longitudinal distance from the panel edge to which the parallel longitudinal cuts were made the combination of the three aforementioned cuts results in severing the internal vertical separating walls within the panel to the desired longitudinal depth the severed vertical walls are removed to leave portions of the flexible upper and lower panel walls available for conformal joining to the upper and lower outer surfaces of the manifold as is shown in fig3 and 5 the upper panel wall 18 and lower panel wall 20 may both be extended beyond the outer edge of the oval manifold tube 25 to form a lap joint between the upper and lower walls the lap joint thus formed constitutes a flange running substantially the full width of the panels and may be punched through and fitted with grommets 28 adapted to fastening the panel to a supporting surface to increase the flexibility of the slit upper panel wall the triangular ridges thereupon may be severed flush with the surface forming their bases as shown in fig5 the slit upper wall 18 and lower wall 20 may be positioned longitudinally with respect to the manifold tube 25 so as to place the sidewall 26 of the manifold in direct.