My Washer/Dryer vent had a missing flap and I couldn't find any replacement flap in the market. The replacement cover costs $18 while a replacement flap would cost less than a dollar to print. The size of the vent pipe was 6". The flap size is 6.25x2.25" or 165x59mm.
Vent_flap_6.5x2.25_in.stl | 45.2KB |