Method for forming, in a metal sheet, a female attachment portion (91a, 91b, 92a, 92b) having an internal ridge, the method comprising: cutting through the metal sheet thereby forming a recess, the recess having a longitudinal extension along a longitudinal geometric axis from an entrance portion to an inner end portion and having a transverse extension along a transverse geometrical axis, wherein the longitudinal and transverse axes (L, T) extend along a major plane of the metal sheet and orthogonal to each other, wherein the metal sheet has free edge portions (12a, 12b) at the entrance portion of the recess, the free edges (12a, 12b) each having an extension (12a', 12b') along the transverse axis and each facing away from the inner end portion, folding a recess surrounding portion of the metal sheet from a first major surface of the metal sheet in a direction transverse to the major plane of the metal sheet through an original extension of a second major surface, opposing the first major surface, thereby forming a folded recess surrounding portion, wherein the recess surrounding portion at least partly includes the free edges (12a, 12b) and at least partly extends around and defines the recess, wherein at least an outer portion of the folded recess surrounding portion extends in a first inclined direction from a portion surrounding the recess surrounding portion, the first inclined direction having a component (ID1') in the transverse direction from the major plane and a component (ID1") along the major plane inwards into the recess, characterized by further folding an inner perimeter portion of the folded recess surrounding portion, the inner perimeter portion at least partly defining the recess, in a direction (T, L) along the major plane of the metal sheet and away from the recess thereby forming a folded inner perimeter portion, wherein the folded inner perimeter portion extends in a second inclined direction from the outer portion of the folded recess surrounding portion, the second inclined direction having a component (ID2') in the transverse direction from the major plane and a component (ID2") along the major plane away from the recess, whereby the thus folded recess surrounding portion, including the outer portion extending in the first inclined direction and the inner perimeter portion extending in the second inclined direction, forms said internal ridge of the female attachment portion (91a, 91b, 92a, 92b). cutting through the metal sheet thereby forming a recess, the recess having a longitudinal extension along a longitudinal geometric axis from an entrance portion to an inner end portion and having a transverse extension along a transverse geometrical axis, wherein the longitudinal and transverse axes (L, T) extend along a major plane of the metal sheet and orthogonal to each other, wherein the metal sheet has free edge portions (12a, 12b) at the entrance portion of the recess, the free edges (12a, 12b) each having an extension (12a', 12b') along the transverse axis and each facing away from the inner end portion, folding a recess surrounding portion of the metal sheet from a first major surface of the metal sheet in a direction transverse to the major plane of the metal sheet through an original extension of a second major surface, opposing the first major surface, thereby forming a folded recess surrounding portion, wherein the recess surrounding portion at least partly includes the free edges (12a, 12b) and at least partly extends around and defines the recess, wherein at least an outer portion of the folded recess surrounding portion extends in a first inclined direction from a portion surrounding the recess surrounding portion, the first inclined direction having a component (ID1') in the transverse direction from the major plane and a component (ID1") along the major plane inwards into the recess, characterized by further folding an inner perimeter portion of the folded recess surrounding portion, the inner perimeter portion at least partly defining the recess, in a direction (T, L) along the major plane of the metal sheet and away from the recess thereby forming a folded inner perimeter portion, wherein the folded inner perimeter portion extends in a second inclined direction from the outer portion of the folded recess surrounding portion, the second inclined direction having a component (ID2') in the transverse direction from the major plane and a component (ID2") along the major plane away from the recess, whereby the thus folded recess surrounding portion, including the outer portion extending in the first inclined direction and the inner perimeter portion extending in the second inclined direction, forms said internal ridge of the female attachment portion (91a, 91b, 92a, 92b).