101 formed on an upper portion thereof and formed by an one ply sidewall having a slot 102 and is unfit for joining the support unit 20 thereto using a fastening screw 213 having a sharp tip portion 214 since a lower portion of the projecting jaw portion 101 is relatively weak for this reason typically a blunt ended tightening screw is mounted to a lower end of the projecting jaw portion 101 in such a way as to be caught to the lower end of the projecting jaw portion 101 so that a joining portion 211 is not separated from the projecting jaw portion 101 see fig1 as shown in fig3 and 4 each of the support units 20 includes a vertical body 200 a horizontal plane portion 201 bent to one side and formed at an upper portion of the vertical body 200 a first screw hole 202 formed on the horizontal plane portion 201 a fastening screw 203 joined to the support unit 20 and the support bar 30 through the first screw hole 202 and having a tip portion 204 penetrating the support bar 30 support portions 205 projecting from the horizontal plane portion 201 a pressurizing member 206 having both end portions which are axially mounted to the support portions 205 the pressurizing member 206 having pressure portions 207 pressing the support bar 30 when the pressurizing member 206 is laid down a rectangular hole 208 formed beneath the horizontal plane portion 201 for allowing an insertion of the support bar 30 an elastic member 209 having upper portions hingedly joined to a central portion of the vertical body 200 and a bent portion 210 formed on a lower portion thereof to clamp a lower end of the projecting jaw portion 101 a.