the score line 32 while a slot 38 is cut in the upper edge of the rib panel 30 centrally therein for receiving the center tab 36 end panels 40 and 42 are hinged at respective score lines 44 and 46 to the ends of the bottom side panel 10 and have upper edges joined by score lines 48 and 50 to overlapping panels or flaps 52 and 54 locking tabs 56 and 58 are hinged at a score line 60 on the inside edge of the overlapping panel 52 while locking tabs 62 and 64 are hinged at a score line 66 on an inside edge of the overlapping panel 54 slots 68 and 70 are formed in the respective top panels 20 and 22 perpendicular to the longitudinal edges of the panels 20 and 22 and parallel to the overlapping panel 52 the slots 68 and 70 are spaced from the left end of the panels 20 and 22 by about the width of the overlapping panel 52 for receiving the locking tabs 56 and 58 similarly slots 72 and 74 perpendicular to the longitudinal dimension of the core are formed in the top panels 20 and 22 spaced by about the width of the overlapping panel 54 from the right end of the panels 20 and 22 for receiving the locking tabs 62 and 64 the slots 68 70 72 and 74 extend substantially completely across the width of the respective panels 20 and 22 the locking tabs 56 and 58 each extend slightly less than half way across the.