A blank for constructing a three-dimensional form is provided. The blank includes a rhomboid panel with three edges and two fold lines, which define a pair of triangular panels. Another rhomboid panel connected to the first rhomboid panel along the common fold line, the other rhomboid panel having three edges and a fold line, which together with the common fold line define another pair of triangular panels. At least one securing flap is connected to one rhomboid edge, the rhomboid panels being foldable towards each other about the common fold line. The pairs of triangles are foldable towards each other and the securing flap is foldable over and connectable to one of the second rhomboid panel edges. A three-dimensional form is also provided.