is molded from lexan brand polycarbonate or a similar product the top panel 12 and the bottom panel 14 each include two grooves 30 and 31 on their interior surface on each panel one of the grooves 30 is proximate to the unflanged front margin and the other groove 31 is proximate to the unflanged rear margin the groove 31 at the rear of the top panel 12 is omitted from fig1 for clarity the grooves 30 and 31 are formed integrally with the panels 12 and 14 by protrusions such as ridges 30a and 30b forming groove 30 and ridges 31a and 31b forming groove 31 raised from the panel surface the top panel 12 and the bottom panel 14 also include a plurality of nibs 32 formed integrally on the interior surfaces of the panels the nibs 32 on the interior of the top panel 12 are omitted from fig1 for clarity the top and bottom panels 12 and 14 may be the same molded part and differ only in the finishing details such as the threaded inserts 42a in the fastener bosses 43 of the top panel 12 or the holes 44 in the bosses fastener 45 of the bottom panel 14 as described below the side panels or members 18 and 20 each have an h shaped cross section as best seen in fig2 the sides of the h form interior walls 18i and 20i and exterior walls 18x and 20x.