A device for altering the airflow pattern from a ceiling vent diffuser by obstructing openings in a region of the diffuser. The device comprises a flexible member that is secured to a diffuser vane by a hook member and two tabs. The hook member has two or more engagement regions which enable it to engage vanes on different manufacturer's diffusers. The first end of the hook member is either pivotally secured to the flexible member or is engaged in an adjustment mechanism thereon. The adjustment mechanism allows the second end of the hook to be situated in different positions relative to an interior edge of the flexible member so as to accommodate different diffusers. An extension detachably engages the flexible member to increase the size thereof to accommodate larger diffusers.