Abstract: An attachment for an electrical outlet box, junction box, or similar receptacle is swiftly, manually engageable in an opening of the outlet box. The attachment is inexpensively formed, preferably of a thermoplastic material, and is of a design lending itself to manufacture by the extrusion process to further reduce the cost of making it. The attachment may be of single or double form, and typically includes a resiliently flexible clamping leg one end of which is integrally formed with a mounting means readily engageable in the mounting opening of the outlet box. The other end is integral with a clamping head, that at least partially overlies a cable entrance, and is formed with serrations that bite into the surface of non-metallic, sheathed cable. When a cable is inserted it deflects the clamping head and leg against the spring tension of the leg.