The invention relates to a shaping method that first comprises preparing a perform (3) by T-punch cutting in a plate or a strip of a cellular foam coil, then transforming said preform (3) so that it takes a shape corresponding to the final part (1), i.e. a polygonal or nearly cylindrical shape, depending on the case. The preform (3) has an outline that fits in a rectangular imprint, and it is cut for forming a slot (6) in the middle longitudinal portion thereof while keeping a portion of material at each end which is sufficient to be used as a hinge (7); it further comprises deep V-shaped notches (8) on its outer outline while also maintaining at said slot (6) a portion of material that is sufficient to be used as a hinge (9).