corner of the cover recesses 32 are positioned above and provide clearance for a pair of similarly positioned locking dimples 34 formed inwardly on the upper portion of flange 29 approximately 1 inch from the center of each of the corners each of the dimples constitutes a flattened semi circular seating surface 35 gripping the under edge of lip 18 and a semi hemispherical body 36 thereunder permitting the dimples to be easily snapped over the lip 18 of tray 10 in a camming fashion the distance between the top surface 35 of the dimples 34 and the bottom surface 38 of ledge 28 is approximately equal to the diameter of the rounded lip 18 on the tray so as to securely grip the lip between the eight dimples two on each corner and bottom surface 38 of ledge 28 of the cover recesses 32 provide the desired clearance for deflecting the corners of the tray downwardly until the cam action dimples snap over the edge of lip 18 providing a tight compression fit of the cover over the lid spaced at predetermined intervals along the sidewalls and end walls of the cover 20 as best seen in fig5 are plurality of integrally formed tapered projections 39 which extend downwardly from the walls 24 and 26 of the tray 10 the flange 29 and projection 39 together with ledge 28 form a generally u shaped construction which circumscribes a portion of.