plate portion 90 has a forwardly extending tab 102 spaced slightly above the plate 90 to capture the enlarged portion 74 of the return flange 72 of the top panel 60 the assembly of parts is shown in detail in fig2 which illustrates the bracket 88 secured to the top 48t and front 48f flanges of the side panel 40 the top flange 62 of the front panel 14 overlies the projecting lip 92 of the bracket 88 and has an aperture 104 therein to receive the screw 100 to secure the front panel 14 to the bracket 88 and thus to the side panel 40 an identical arrangement is provided at the side panel 38 and bracket 86 the top panel 60 is placed on the top edge flanges 46t 48t of the side panels and the top flange 62 of the front panel such that a front edge 106 of the top panel is extended forwardly of the front panel 14 the top panel 60 is then slid rearwardly until the rear edge 64 slides under the forwardly projecting flanges 58 of the top rear panel 56 and the enlarged portion 74 of the return flange 72 slides beneath the tab 102 of the bracket 88 in this manner both the rear edge 64 and front edge 106 of the top panel 60 are secured against vertical movement the flanges 72 acting against tabs 102 prevent horizontal movement of the top panel front portion two or three threaded fasteners such as screws 108 can then be secured through the flanges 58 and apertures 66 along the rear edge 64 of the top panel to prevent the top panel from sliding.