A post construction wall mounting box assembly (100) is formed from an enclosure (200) and a clenching bracket (300) which are joined together with joining screws (431, 432) where the enclosure includes a first set of parallel and opposed sidewalls (210) having exterior wall flanges (214) and the clenching bracket includes first and second slideable sidewall portions (310, 320) that have interior wall flanges (314, 324) and these exterior and interior wall flanges cooperate to engage an edge of a wall board when the joining screws are tightened.