A system for mounting a prefabricated construction unit in an aperture in a building structure comprises a plurality of clips. The clips have exterior ends adapted to be affixed to a frame of the construction unit and interior ends adapted to be affixed to the building structure. The exterior ends may be bent to provide deep grooves which receive a flange of the construction unit. The clips permit a construction unit to be installed without the need for a worker to insert fasteners through the flange from outside the building structure.