A support ring has an oval upper edge and an oval lower edge. The lower edge is smaller than the upper edge. A tapering oval sidewall is provided between the upper and lower edges. A flange is formed in the support ring. The flange extends outwardly from the upper edge. A groove is provided in the side wall. The groove is provided essentially midway between, and parallel with, the upper and lower edges. A plurality of similarly sized and shaped ramps are formed in the side wall between the groove and the lower edge.