as aluminium tubing or the like the first second and third legs 14 16 and 18 include first ends 14a 16a and 18a and second ends 14b 16b and 18b as may be seen in fig1 first second and third tubular extensions 14c 16c and 18c project upwardly from the first ends 14a 16a and 18a of the legs and these extensions including first second and third end portions 14d 16d and 18d that are straight and vertically disposed relative to the ground surface when the invention a is disposed as shown in fig1 each of the inventions a includes first means c for pivotally connecting the first second and third legs 10 12 and 14 together as shown in fig1 the first means c includes a rigid body that has first second and third circumferentially spaced arms 20a 20b and 20c defined thereon the first second and third arms 20a 20b and 20c have bifurcated free ends that serve to support first second and third pins 22 24 and 26 therein and these pins passing through transverse bores formed in the first end 14a 16a and 18a of the first second and third legs 14 16 and 18 as shown in fig5 first second and third clamps 28 30 and 32 are provided that are in the form of rollers that are rotatably supported on the upper extremities of the first second and third end portions 14d 16d and 18d as may be seen in fig3 with the rollers having first second and third cylindrical surfaces 28a 30a and 32a each of the first second and third clamps 28 30 and 32 includes an upwardly disposed longitudinally extending recess 34 that is defined by a flat.