Abstract: An auxiliary or primary lid (10) for sealing a container (12) having a rectangular cross-sectional configuration including four right angle corners is provided. The present lid includes a flat rectangular top portion (32) which corresponds to the cross-sectional configuration of the container to be closed and a continuous skirt portion (50) having four planar side portions (42, 44, 26, 48) joined perpendicularly to the perimeter of the top portion to form four right angle corners (52, 54, 56, 58). Formed in the interior surface of each planar side portion is a pair of outwardly projecting ribs (64, 66, 70, 72, 76, 78, 82, 84), each pair being separated by a discontinuity (68, 74, 80, 86) positioned in the center of each planar side portion. The ribs in adjacent planar side portions intersect at the corners of the skirt. Each corner includes, in addition, corner reinforcing means for stabilizing the corner and enhancing the gripping properties of the corner.