102 and a rope is able to tie at the tie portion 13 a pivotal portion 14 projects from a lateral side of the base member 10 between the first and second ends 101 102 a positioning hole 15 pierces through the pivotal portion 14 longitudinally and includes a first section 151 a second section 152 and an annular groove 153 transversely defined therethrough between the first and second sections 151 152 and communicating with positioning hole 15 a piece 16 is adapted to insert into the annular groove 153 first second and third radial position portions 154 155 156 are defined in the second section 152 shown in fig5 a cross section of the second section 152 of the positioning hole 15 is a circle with three radial slots and the slots are not in the same radial position the pivot plate member 20 which has first and second ends 201 202 includes a receiving recess 21 and first and second positioning openings 22 23 the receiving recess 21 is adapted to receive the pivotal portion 14 so that the pivot plate member 20 is able to pivot with respect to the pivotal portion 14 the first and second positioning openings 22 23 respectively correspond to the positioning hole 15 a through hole 24 pierces through the second end 202 longitudinally a fastening member 25 is inserted through the through hole 24 and includes a handle 251 defined at an end thereof and a threaded portion 252 formed on.