A stiffening system for the perimeter edges of a foam core mattress comprising a coil spring structure disposed along one or more perimeter edges of a mattress having a foam sleeping area. Such capacitor may be useful in space restricted applications, and in applications that require very close electrical connections between the power consumer and the power supply. The second layer superimposes a region of the first layer. The locking mechanism can include a locking nut having two non-concentric cylinders which enable an interference fit between the nut and the post, thus restricting axial translation of the second plate.