A molding apparatus (10) for forming a retaining device. The molding apparatus (10) comprises a molding strip (12) and a molding support (24). The molding strip (12) has an inside face (14), an outside face (16), and a plurality of through cavities (18) extending from the outside face (16) to the inside face (14), the molding strip (12) extending in a longitudinal direction (X) and presenting both a transverse direction (Y) perpendicular to the longitudinal direction (X), and also a height direction (Z) perpendicular to the longitudinal direction (X) and to the transverse direction (Y). The inside face (14) is configured to press against a molding face (26) of the molding support (24), wherein the inside face (14) of the molding strip (12) and/or the molding face (26) of the molding support (24) includes an array of passages, the array of passages forming vents and connecting together the cavities (18) when the molding strip (12) is pressed against the molding support (24).