Depends on what all you want your UDS to do. With my flat lid I could check the meat temps without removing the lid, look at the meat to see how it looked, rotate the exhaust opening to adjust where more heat is needed ( where there is draft/flow, there is more heat for the most part). I found a chimney to be a plus on windy days. I had the holesaw, the aluminum flashing, screw, washer and used coffee can.
I also found controlling the outlet is second to controlling the inlet..actually just controlling the air flow period.
But then again I run my UDS from 180 to 700+ degrees