The invention relates to a piece for placing against a face of a bodywork element, which element includes on said face a fastening projection made of plastics material. The piece has a fastening portion made of plastics material that is configured to form a tab which, when the piece for placing against the bodywork element is in position, extends parallel to the projection and can be secured to said projection by material melting. The invention also provides a bodywork element suitable for receiving such a piece, and an assembly constituted by the piece and the element.