An enclosure such as an electrical junction box has fixed clamping flanges exterior to the enclosure, screws rotatable relative to the enclosure and operative for drawing clamping brackets from an initial elevated position towards the clamping flanges for clamping the edges of an opening cut in drywall or the like, thereby to fasten the enclosure inside a wall or ceiling. The clamping brackets are initially retracted in the enclosure and rotation of the screws releases the brackets to an extended clamping position relative to the clamping flanges.