fig5 cut into chamber 62 with the outer edge 72 of slot 70 cut into shelf 64 and the inner edge 74 of slot 70 cut into indentation 68 as can be best seen in fig5 slot 70 may be cut by a knife die or punch moving in a vertical plane i e perpendicular to top wall 12 the width of slot 70 is such that when viewed in the vertical plane fig2 slot 70 is substantially wider than the thickness of central segment 52 typically slot 70 may be about 3 times the thickness of central segment 52 it should be noted that if such a slot were cut in side wall 14 rather than shelf 64 the width of slot 70 when viewed in the vertical plane would be many times the preferred width which would substantially decrease the structural integrity of the container and increase the probability of the contents and or vapors escaping from the container in fig8 a modified slot configuration is shown which is the preferred form of this invention the slot 70a is generally u shaped in plan view as is the slot 70 shown in fig7 however unlike the slot 70 slot 70a is defined by straight inner and outer edges 100 and 102 straight inner and outer side edges 104 and 106 and straight front edges 108 the several edges are not joined by radii but rather meet at square corners consequently the punch and die used to cut the slot 70a is made with readily machinable parts the clearance between the punch and.