I am assuming you have three intake holes (you didn't say).....
If so, leave the top exhaust vents open (think of them as the exhaust pipe on a car - if you plug them, the exhaust has nowhere to go, and your engine will fizzle), and open all three intake ports.
As your UDS comes within 10-20 degrees of target, close off ONE intake. Watch the temp. When it gets within 5 degrees of target, close the SECOND intake. Watch the temp again. It *should* stay at the desired temp. If not (and this is where ball valves and magnets earn their pay), restrict the air flow to the THIRD intake.
This should put you on track, unless you have a leak. If you do, use foil, gasket rope, a welders blanket, or RTV caulk, depending on where it is.
Dbl Bbl Smoker, UDS, ECB, Char-Broil gas grill & charcoal grills, SJ - Certified Moink Baller