As seen in FIGS. 7 and 8, the mounting rail is a generally U-shaped configuration with a bottom wall 73 and sloping side walls or edge flanges 74 which are inclined upwardly away from the bottom wall 73 and terminate at an outwardly projecting flange portion 75.More particularly, the bottom wall 73 includes an interior face 76 which has a pair of upstanding dove-tailed shaped connector ribs 77.