to prevent some conformity with the slightly variable shape of the opening 18 or to require excessive separation between the cushion portion 14 and the back portion 16 for permiting insertion of the front portion 26 into the passage 18 for this purpose a thickness t of between 0 2 and 0 3 inch have proven satisfactory more preferably the thickness t is approximately 0 25 inch in order to effect a rigid connection between the u shaped member 28 and the anchor member 24 the apparatus 20 incorporates an interlocking fastening means 50 between the u shaped member 28 and the leg members 30 as best shown in fig1 and 3 the fastening means 50 includes a pair of head members 52 extending from the front of the front leg portion 34a of the u shaped member 28 the head members 52 engaging respective cavities 54 that are formed in the leg members 30 the leg member 30 being formed of the same relatively flexible material as the remainder of the anchor member 24 is capable of deforming sufficiently to accept the head member 52 a neck portion of 56 of each cavity 54 fixably engaging a shoulder portion 58 of each head member 52 upon full engagement of the head member 52 with its respective cavity 54 thus the u shaped members 28 can be quickly and easily attached rigidly to the anchor member 24 without the requirement of separate fasteners or the impractical bonding of dissimilar materials instead assembly is effected by the simple expedient of pressing the parts together a plurality of tooth shaped protuberances 60 are formed on the bottom of the anchor member 24 for maintaining the apparatus 20.