Abstract: A unitary, bent corner piece for a valance frame has an extending tongue at each end shaped to interfit the hollow cross-section of an adjacent, extruded, valance frame member. Complementary corner pieces so formed provide part of a valance interface between the lid and body of a container, ensuring sealing continuity around corners fitted with such corner pieces. Each corner piece is provided with a mating valance surface for presentation to the valance interface to provide an intimate engagement with the mating valance surface of the complementary, opposed corner piece. Gaskets fitted within the corner pieces may form part of the interface. Tongues formed a at the respective ends of a corner piece are provided with a groove to accommodate the inclusion of a gasket into the interior core of the longitudinal frame members.