radioguy
Master of the Pit
Take a look my mailbox mod. I used 3 inch auto exhaust pipe. I rarely use the butterfly valve. I do use the top flap to control airflow.
https://www.smokingmeatforums.com/threads/one-more-mailbox-mod-works-great.158043/
https://www.smokingmeatforums.com/threads/one-more-mailbox-mod-works-great.158043/