a stacking means four planar sidewalls 175 extend downwardly from the upper lip 178 and along with inset corner portions 173 define a wafer enclosure the wafer enclosure perimeter 179 is generally square in shape at each corner 173 is a flange portion 171 with apertures 148 to receive the latch member 132 top cover 104 in conjunction with the sidewalls 175 provide a top cover 104 that is highly rigid even when formed with such soft plastics as polypropylene the latching apertures 148 in cover 104 are formed in an approximate t shape this shape comprises a first width or head 150 and a second narrower width or neck 152 the neck 152 is slightly greater in width than the width of the latching member 132 where it extends through the neck the head 150 has a width greater than the neck 152 the latching member 132 engages the head 150 of the aperture 148 and is moved into the neck region 152 due to the movement of cam surface 136 along the underside of the cover 104 corner portion 173 during the latching process referring to fig2 and 24 the movement of latching member 132 within the latching aperture 148 is illustrated the latching aperture has a depth dimensions d 1 d 2 and d 3 as shown dimension d 2 is equal to d 1 plus d 3 dimension d 3 is the depth of the engagement surface 138 dimension d 1 is.