extend past the vertical edges of the inner end wall panels 24b and the first and second strengthening panels 28 and 30 to abut the inside surfaces of the outer end wall panels 24a these reinforcing tabs 48 rigidify the corners of the container 12 by preventing the end walls 24 from bending inwardly a vertical slot 50 is formed in each end wall 24 between its inner and outer panels 24a and 24b as best shown in fig5 and 6 these slots being vertically aligned when the containers 12 are stacked to form the slots 50 openings 52 are provided in the floor panel 18 beneath the center of each end wall 24 and a corresponding opening 54 is cut in the top strip 24c of each end wall the vertical edges of the slots 50 are coincident with the ends of the first strengthening tabs 28 adjacent each end of the slot 50 at the top thereof small upstanding protrusions 56 and 58 are formed on the top edges of the inner end wall panels 24b and the second strengthening panel 30 when the containers 12 are stacked these protrusions 56 and 58 fit into the slot 50 of the container above to inhibit relative lateral shifting between the containers even before the plastic inserts 14 have be installed the inserts 14 for the slots 50 best shown in fig3 are each integrally molded as.