and the framework 38 of the base 22 referring to fig5 a receivers 46 are recessed into the exterior surface 62 of the panels 24 the receivers 46 have a recessed circular portion 74 with a through bore 76 at its center an arcuate segment of the circular portion 74 is broken to define a radial opening 78 extending from the center bore 76 through to the bottom edge of the panel 24 each side panel 24 slides over the bolts 44 in the base frame 38 through the openings 78 in the slots 46 as shown in fig5 b 5c the center bore 76 is sized to fit over the bolt 44 and the recess 74 is sized to capture a washer 55 on the bolt 44 when the bolt 44 is tightened this connection prevents the panels 24 from being longitudinally separated from the base 22 to accomplish this shoulders 75 border the recess 74 adjacent the lower edged washer 55 to prevent the side panel 24 from being pulled off of the base 32 when the bolt 44 is tightened ramps 77 lead up to the apex of shoulders 75 to facilitate sliding the receiver 46 under the washer 55 the mouth 79 of the receiver 46 at the lower edge of the panel 24 also flares outwardly to ease.