A container assembly comprises a container having a peripheral side wall defining a rectangular interior space and having top flap and bottom flaps folded inwardly to selectively enclose the interior space. The bottom flaps include notched edges for interlocking to maintain corners of the peripheral side wall square. An insert is removably receivable in the rectangular interior space. The insert comprises first and second panels of sheet material selectively folded to define a plurality of rectangular mold cavities in the interior space. Distal edges of the panels are notched wherein the notched edges interlock at the corners of the peripheral side wall to maintain the mold cavities square.