<p>The system has a fixing unit for fixing two boards (14) to a support (18) and wings (8). Each wing (8) takes support on a horizontal surface of a groove (16) of one of the boards (14) in a point when the boards are fixed to the support (18) by the fixing unit (2). A difference of height (h3) between a lowest point of each wing and a base (4) is smaller than a difference of height (h2) between the top of each branch and base.</p>
