side panel 14 on either side of the tab 22 the pin 30 includes an enlarged head 34 at one end that prevents the pin from passing completely through the opening 24 details of the pin 30 are best seen in fig4 a 4 i in particular the pin 30 is preferably a unitary body 31 and most preferably a molded body in a specific embodiment the unitary body 30 is formed of a plastic material that is inexpensively and easily molded to include the features of the pin described herein the body 31 defines the enlarged head 34 with a generally planar underside 35 that bears against the vertical panel 14 as shown in fig3 in one feature of the invention the body 31 further defines a locking barb 32 that projects at an angle from the planar underside 35 the locking barb 32 is particularly configured to extend into the slot 20 as shown in fig3 to lock the pin in place within the tab opening 24 the locking barb 32 is preferably resiliently deformable so that the barb can pass through the space between the tab opening 24 and the outer face of the vertical panel once the pin 30.