The box is formed as of a rectangular cardboard sheet upon which there are parallel perforated fold lines (1) which determine, in the folding, the formation of a regular octagonal prism with the bases, partially open.Parallel to the edges of the sheet perforated fold lines (6) and (7) have been made forming two strips divided by the perforated fold lines (1), in which divisions there are diagonal perforated fold lines (8) that divide up the strips in triangular areas. Upon the folding of the sheet these triangular areas extend towards the inside of the prism and determine the narrowing and partial closing of the bases of the prism forming a diaphragm.