uniform thickness about half the thickness of the first upwardly extending member 22 the first engagement member 30 is extruded from a polymer having more resiliency that is softer than the polymer from which the planar base 20 and the first and second upwardly extending members 22 24 are extruded a second engagement member 20 extends inwardly and downwardly from the top of the second outwardly extending member 24 the second engagement member 40 has a first planar portion 42 an arcuate portion 44 and a second planar portion 46 the second engagement member 40 is the mirror image of the first engagement member 30 and is fabricated from the same softer material a first corner fin 50 extends downwardly and outwardly from the edge of the planar base 20 adjacent the first upwardly extending member 22 the first corner fin 50 is about u215c inches in length and tapers to be less thick at its remote end 52 when compared to its base 54 adjacent the planar base 20 a similar second corner fin 60 extends from the edge of the planar base 20 adjacent the second upwardly extending member 24 the second corner fin 60 tapers from a thicker thin base 64 to a.