Abstract: A single individually molded plastic unit comprises a base member having two opposite side edges with a flat bottom surface and an open top surface having a raised continuous periphery. A cover member has the same shape and two side edges as the base member, and also has a top surface and an open bottom surface having a raised continuous periphery. One side edge of the cover member is secured to one side edge of the base member by a living hinge. A catch is detachably engageable with the bottom surface of the cover member and is supported by an external projection secured to the opposite side edge of the base member. Inward movement of the projection toward the base member releases the catch engagement and allows the cover member to be pivoted about the living hinge away from the base member.