28 is disposed between and aligned with the tubular portions 32 a pin 34 is inserted through the tubular portions 32 and 28 thus the plate 30 is pivotally attached to the plate 26 an edge which is opposite to the edge along which the tubular portions 32 are formed is shaped as a concave arc two margins 36 beside the arc shaped edge are bent a rubber pad 38 or the like is securely attached to the plate 30 by means of an adhesive or other appropriate means a fastening assembly has a casing 40 having two plates parallel to each other each plate is shaped as a semi circular portion joining an isosceles trapezoidal portion tapered away from the semi circular portion two walls integrally extend along the sides of the isosceles trapezoidal portions of the plates of the casing 40 thus an opening 42 is defined along the semi circular edges of the plates of the casing 40 and an opening 44 is defined along the bottom edges of the isosceles trapezoidal portions of the plates of the casing 40 the first plate of the casing 40 has a slot 46 a disk 48 has a protrusion 50 extending from its center the disk 48 is disposed in the casing 40 with the protrusion 50 slidably received in the slot 46 the.