reinforcement 6 and the other two small holes 11 to these four small holes are connected the tension tubes 13 that give the device its structure the eight tension tubes 13 consist each of a filled or hollow elongated tube filled with expanded polyurethane as shown in fig7 the ends of the tubes 13 are flattened as at 14 perpendicular to one another the flattened ends connect to a small inclined terminal portion which has a central perforation 15 the four tension tubes 13 have their inclined terminal portions oriented toward one side symmetrically opposed to one of the inclined terminal portions of the other four tension tubes 13 each pair of tension tubes 13 with opposite symmetrically inclined terminal portions are joined to a connector made up of two rebushed bolts 16 and a rubber support 17 of two truncated conical sections joined at their smaller bases as seen in fig6 and 7 the larger bases of the rubber supports 17 are joined each to an annular projection from the center of which the two opposed bolts 16 project the bolts 16 extend through the central perforation 15 of the tension tubes 13 and are secured with nuts as seen in fig1 3 the connectors are positioned in the small holes 11 and 12.