sized and shaped to snap together when aligned under external pressure in the preferred embodiment the first member of the pair is a cylindrical post 85 which is sized to be received within the second member cylindrical ring 81 as illustrated in fig3 first inner face 70 of first hollow frame 61 is provided with two cylindrical rings 81 each ring 81 is positioned between peripheral edge 68 and inside edge 67 on first inner face 70 and is located on the side of the rectangle which is distant from fold strip 90 the inner diameter of each ring 81 is sized to receive the post therewithin in a close fit the outer diameter of ring 81 is slightly larger than distance between peripheral edge 68 and inside edge 67 and is therefore flattened along portions which intersect peripheral edge 68 see reference number 83 in fig3 and 4 and inside edge 67 this truncation of the outer perimeter of ring 81 preserves the smooth appearance of casing 60 when assembled second inner face 72 of second hollow frame 63 is provided with two cylindrical posts 85 each post 85 is positioned between peripheral edge 77 and inside edge 78 on second inner face 72 is located on the side of the rectangle which is distant from fold strip 90 and are positioned to mate with the associated ring 81 when casing 60 is folded ring shaped channel 84 is.