This accessory comprises: a plane bottom (2) with a first edge (10) and a second edge (12) parallel to the first edge (10), a first flange (4) inwardly curved from the first edge (10) towards the front of the bottom (2), a second flange (6) inwardly curved from the second edge (12) towards the front of the bottom (2), the first and second flanges (4, 6) being inwardly curved towards one another so that with the part of the bottom (2) they exhibit a C-shaped profiled form, each flange defining a housing for a transverse wire (32), a third flange (8) extending towards the front of the bottom (2), the third flange (8) being disposed in such a way that it defines together with the ends of the first flange (4) and of the second flange (6) on the side of the third flange (8) on each occasion a housing intended to receive a longitudinal wire.