Abstract: An assembly is provided including a front rail, a rear rail, a pair of side rails and a plurality of corner pieces. A pair of self-adjusting devices interconnect the two side rails and the rear rail. Each self-adjusting device includes a stop coupled to a side rail, a biasing device disposed between the stop and a corner piece, and an interconnecting plate fixedly coupled to the side rail and slidably coupled to the corner piece for accommodating relative movement therebetween. According to one aspect of the present invention, the interconnecting plate is coupled to the stop. According to another aspect of the present invention, the biasing device is a coil spring. According to still another aspect of the present invention, the interconnecting plate includes a slot for accommodating sliding movement of a fastener passing through the interconnecting plate and coupling the interconnecting plate to the corner piece.