It is constituted by several plates (1), provided in its outer side (4) with transverse ribs (5), formed by C-shaped profiles and joined together to form a prismatic contour (7), closed by the inner flat sides of all the plates, the plates comprising a means of mutual fixing and detachable means of immobilization. The means of mutual fixing are constituted by hinges (8), fixed to the edges of the plates, and by corresponding oblong orifices (9), evenly distributed on the plate (1) in transversal direction. The means of immobilization are constituted by a through hole (11) made in the shaft of each hinge (8) and by a sliding element (12) provided with a wedge-shaped end (13).