shown in perspective in fig6 are formed in cutouts 34 at two locations along the rib 28 at corresponding locations in the channel 24 cutouts 36 are also provided above each cutout 36 is a projection 38 see fig3 when the rib 28 is inserted into channel 24 the projection 38 pushes the cantilever tab 32 backwards into the cutout 34 however as the rib 32 continues to be inserted into the channel 24 the top of the cantilever tab 32 is eventually below the level of the projection 38 and as a result of its resilience springs out and engages the underside 40 of the projection 38 thereby holding the rib 28 in the channel 24 this arrangement is shown in fig5 of the accompanying drawings a further edge channel 26 is located along one of the shorter sides of the rectangular element shown in fig1 and a cutout 36 including a projection 38 is located part of the way along its length along the shorter side of the element opposed to channel 26 a further portion 42 is provided that is connected to the first portion 12 by a hinge 44 that is identical to the hinge 16 the portion 42 also includes a downwardly extending rib 46 that is similar to rib 28 and extends along distance 34 k 34 shown in fig1 along rib 46 there is provided a cantilever tab 48 in a cutout 50 of a design corresponding to tab 32 and cutout 34 adjacent elements can be joined side by.