A fastening device may include, in some embodiments, two end parts, each of which has a bearing face for resting on a flat support and which can be assembled to each other by a sliding connection with marked positions. A cradle of the device, for accommodating an elongate object transversely to the flat support, has two half-cradles respectively carried by the two end parts. The two end parts have a structure for catching onto side edges of the flat support.