I've built the Double Barrel. I went with 1 6" exhaust in the center of the top. I did not put a damper on it and never needed one. It taught me to control my fire. YMMV.
Feel free to PM me with any questions. I've got some things that I'd definitely change if I built another one.
Jet Coatings Smokin' & Fishin' Crew
Indian word meaning lousy hunter.
Did someone say ROAD TRIP?!?!
NB offset with baffle and chimney mods--STOLEN
Double-barrel 55-gal drum smoker
A farking weed burner...hehe hehe! Fire's Cool!