36 in the center of the window panel 26 two large c shaped slots 38 in the end panel 14 and a small round slot 40 across the fold between the back panel 16 and window panel 26 on the other hand between the cover panel 30 and end panel 22 a rectangular sleeve insert 42 may be bonded to inner surfaces of the panels near their distal edges the sleeve insert 42 may be a thin paper pocket with a middle fold line 44 and two side closing flaps 46 an elongated entrance 48 is formed at the remaining side of the insert 42 in order to facilitate correct positioning of the insert 42 between the cover panel 30 and end panel 22 the insert 42 is provided with an enlarged edge 50 which is sized to match the distal end portions of the panels 22 30 when they are permanently bonded together an end fold 52 of the end panel 22 helps keep a planar space for the sleeve insert 42 between the panels 22 30 and provides a smoother edge that is safe to the touch an elongated slit 54 with rounded ends 56 is formed in the end panel 22 at a position to cover the entrance 48 of the sleeve insert 42 in position during assembly the exterior surfaces of the entrance 48.