An end unit has an end wall and first and second side walls forming a corner. One end of an L-shaped rail with first and second flanges seats in the corner against the end wall, with the flanges flat against the side walls. One side wall has a protrusion abutting the first flange to restrict movement of the rail away from the other side wall. The first side wall has a tab in a slot in the first flange, engaging the flange on a side of the slot away from the end wall, preventing movement of the rail away from the first side wall, while permitting movement of the rail away from the end wall to a position where the tab aligns with the slot to permit movement of the rail away from the first side wall. A pin in aligned holes prevents the movement away from the end wall.