An assembly for covering a grill of an air conditioning vent in the winter comprises a cover having a frame that may have a hole on a top surface and sides including a compressible material along its perimeter for sealing a vent area. In one preferred embodiment, the assembly includes an elastic cord having a looped end and a free end, the looped end being of adjustable size and having two hooks slidably connected to the cord. The hooks are configured to attach to the grill and pulling the free end of the cord tightens the looped end and the hooks. The free end may be attached to the cover so that sliding a barrel lock up the cord urges the cover against the vent area and prevents unwanted upward hot air flow and downward cold air flow in the winter when the air conditioning system is not in use.