3 and b 4 that are identical in structure and each of which has a first end 10 and second end portion 12 each elongate member b 1 b 2 b 3 and b 4 has first and second opposed side surfaces 14 and 16 and first and second edge surfaces 18 and 20 first second third and fourth heads c 1 c 2 c 3 and c 4 are provided that are of identical structure and are preferably formed as integral parts of the first second third and fourth elongate members b 1 b 2 b 3 and b 4 on first ends 10 thereof as may be seen in fig3 the first second third and fourth heads c 1 c 2 c 3 and c 4 are of identical structure and accordingly only the structure of the first head c 1 will be described in detail head c 1 includes a transverse channel shaped member defined by a web 22 and first and second laterally spaced flanges 24 and 26 that extend outwardly from the side edges of the web as may be seen in fig1 the first and second flanges 24 and 26 have first and second axially aligned openings 24a and.