130 the cover 150 is made of injection molded plastic and is sized and configured to effectively seal off the hollow interiors 139a and 139b of the sill frame member 130 a mirror image of the cover 150 is provided for an opposite cut end of the sill frame member 130 the cover 150 is shown in greater detail in fig4 7 the cover 150 includes a first or main wall 151 having an inwardly facing surface 152 the perimeter of the first wall 151 corresponds to the perimeter of the hollow interiors 139a and 139b a slot 153 is formed in the first wall 151 to coincide and align with the groove 137 on the sill frame member 130 a notch 154 is formed in the first wall 151 to coincide and align with the screw chase 136 on the sill frame member 110 prongs 155 are integrally connected to the inwardly facing surface 152 the prongs 155 extend perpendicularly inward therefrom and cooperate with the sill frame member sidewalls to align the cover 150 with the cut end 138 when the former is inserted into the latter a screw chase or bridge 156 is also integrally connected to the inwardly facing surface 152 the screw chase 156 spans the height of the relatively outward interior 139a as defined between the lower sill wall 133 and the screw pilot 136 the screw chase 156 also provides a vertically oriented hole through which a fastener is inserted to secure the side frame member 110 to the sill frame member 130 and the cover 150 an energy director in the form of a ridge 158 is positioned on the first wall 151 extending parallel to and just inside the perimeter thereof.