convenient to use them as correct positioning means of the stiff members 7 9 11 they additionally facilitate positioning of the trays 3 3 u2032 with the suitable inclination the alternating arrangement of trays 3 3 u2032 defines a triangular prismatic shape of the device 1 in the preferred embodiment shown in fig2 a tray 12 formed by two half trays 14 15 equal with the exception of the position of the supports 30 31 can be seen half tray 15 has a broken surface form with an upper portion 17 and a lower portion 19 that are parallel and tilted at an acute angle i with regard to the horizontal plane and a central almost horizontal portion 21 the upper portion 17 and lower portion 19 include regularly arranged open areas 22 it is calculated that the suitable inclination of portions 17 and 19 with regard to the horizontal angle i in fig2 must be comprised between 40 u00b0 and 80 u00b0 and that of the central portion 21 angle c in fig2 must be comprised between 0 u00b0 and 15 u00b0 the inclination of portions 17 and 19 can be different the half tray 15 is supported at one of its ends on the triangular type support 31 with a broken side such as the surface of the tray 15 with an.