corner portion of the receiving surface 20 of the small diameter portion 22 indentation parts 26 are formed in the outer circumference wall 12 b of the base 12 along the perimeter direction with a prescribed interval in between with the indentation parts 26 a perimeter wall is prevented from sinking after molding and stay parts 27 are formed between indentations 26 s for strengthening the base 12 a clip 28 shown in fig2 and 4 a to 4 c is made of nylon and provided with a cylindrical shape main body 30 capable of inserting the small diameter portion 22 so that the clip 28 can be inserted into an installation opening 18 a formed in the protective plate 18 a pair of plate pieces 32 is provided on the main body 30 and capable of tilting relative to the main body 30 at tips of the plate pieces 32 claws 34 project from an outer circumference surface of the main body 30 when the main body 30 is penetrated through the installation opening 18 a of the protective plate 18 the claws 34 contact the installation opening 18 a and the plate pieces 32 tilt to reduce a diameter so that the claws 34 can be penetrated through the installation opening 18 a after the claws 34 penetrate through the installation opening 18 a the claws 34 and.