1964 Plymouth Valiant Heater Box Door

Hey all,

I'm looking for what I think is called a heater box door for my '64 Valiant. There is a missing 'door' approximately 5'' or so on an assembly under the dash by the clutch pedal that is constantly blowing air on my left foot. Not sure why the need for a door, just know I want to seal it up with a factory part. Thanks.