Each of the divider walls 220 are shown in FIG. 2B having a bend or bow across the front edge 226. As shown in FIG. 2B, the bend or bow is notably identified with respect to a dashed line 271 that is level across the width of the frame 210 from one corner edge 222 to the other corner edge 224, shown from the side, with the dashed line 271 indicating a nominal position of the divider wall 220 if no bowing were present. Note that the bowing depicted in FIG. 2B is exaggerated to make it more visually apparent, and actual dimensions are not depicted.