114 and the flanges 114 taper toward one another in a direction that extends from the web 112 to distal edges 116 the flanges 114 generally define planar inner surfaces 118 inclined toward one another and extending between the web 112 and cantilevered lips 122 lips 122 generally define planar inner surfaces 124 the inner surfaces 118 of the flanges 114 and the inner surfaces 124 of the lips 122 form corners where they meet the carrier is preferably an integrally molded polymer but is structured to be strong and substantially rigid i e only minimally resiliently deformable a useful feature of the embodiment of fig7 a is that shoulders 126 are formed in the corners between the cantilevered lips 122 and the flanges 114 each shoulder 126 is built up in two parts a wedge of material 128 extends from a line 132 on lip 124 that is midway between inner sharp edges 134 and outer base ends 136 in the flange 114 also an enlargement 138 forming a reinforcing bead or bulge extends from line 132 on lip 124 at the wedge 128 to a line 142 on the flange 118 that is about midway between lip 122 and web 112 the relatively larger second part 138 of the shoulder 126 defines a convex surface in section fig6 shows that wedge part 128 extends from end to end of carrier 104 the enlarged part 138 terminates at a point inwardly spaced from the ends of the carrier shoulder 126 including both parts 128 and 138 reinforces lips 122 against deformation due to forces acting in the direction of arrows 144 an open channel 146 is formed.