13 the notches 8 define a plurality of adjacent vertical rectangular edge portions 20 22 24 26 on the right side of each panel 7 13 and a similar plurality of edge portions 19 21 23 25 on the left side of panel 7 the panels 7 13 are fastened or interlocked to one another by joining the panels at their respective notches 8 in such a manner that the opposing rectangular edge portions of each panel alternatively overlap one another in a serpentine fashion in particular the lower surface of portion 19 grips the upper surface of portion 20 the upper surface of portion 21 grips the lower surface of portion 2 the lower surface of portion 23 grips the upper surface of portion 24 and the upper surface of portion 25 grips the lower surface of portion 26 beneficially when the notches 8 and opposing rectangular edge portions of each panel are interlocked they resiliently deform and stretch the internal crisscrossed b flute corrugations not shown of the panels because the panel corrugations have a plastic memory the slight deformation resulting from the intertwining of the edge portions causes continuous stress and tension in the material that not only helps to maintain the junction but also stiffens each of.