Abstract: A modular closure unit is described which has three walls, two of which are side walls each connected to the opposite vertical edges of the third or back wall. A top member is also integrally attached to the top edges of all three walls. Two discrete attachment means detachably attach the unit to the doorframe of an existing doorway in a wall to thereby close the doorway and prevent the unilateral removal of the unit by persons having access to only one side of the unit. The unit permits operable retention of the previously mounted door when desired.