245 extend along opposite sides of the frame 241 when the front panel 240 is folded toward the rear panel 220 and into substantially parallel relationship therewith the two panels 240 and 220 cooperate to form a parallelepiped housing bordered by the sheets 241 and 221 the end walls or ridges 222 and 223 and the side walls or ridges 244 and 245 the front panel 240 borders a rectangular opening 246 in the front face of the parallelepiped housing a plurality of posts or stakes 250 are disposed about the perimeter of the frame 241 and protrude rearward therefrom toward the rear panel 240 for reasons discussed below also two prongs or latches 252 are disposed at the end of the frame 241 opposite the hinge 239 and protrude rearward as well both the prongs 252 and the posts 250 are integrally molded to the frame 241 a profile of one of the prongs 252 is shown in fig5 each prong 252 includes a relatively narrow stem 254 and a relatively larger head 255 the head 255 cooperates with the frame 241 and the stem 254 to define a notch 258 between the head 255 and the frame 241 an angled surface 256 is disposed on the head 255 and faces in the same general direction as the notch 258 as the front panel 240 is folded toward the rear panel 220 and into substantially parallel relationship therewith each of the angled surfaces 256 engages an edge of a respective slot 232 thereby causing a respective stem 254 to yieldingly bend as either head 255 fully penetrates a.