I just made some mods to my UDS. I had a normal 3 intake system with 1 of them being a ball valve. I used the bung hole as the exhaust. For a while I been smoking at 225* and all was well. Until I tried getting my temps higher and was having a really tough time. So I added a 4th intake. Went to HD and found a threaded pipe, think it was 2" diam. and cap. I also screwed a 4" length exhaust with a cap to the bung hole in the lid.
Last night I spatchcocked 2 chickens and had that baby at 375-400 with no issues at all. Was no problem with the big intake and the ball valve at about 1/2 way.
Anyone who is is building an UDS right now I wud highly recommend adding this larger 4th intake and use a draft.
One more thought regarding a water pan. I will never use one again. 2 weeks ago I made some ribs (3 BB racks). Took the membrane off and used a nice coating of rub on them. Smoked them at 275-300* (I think.. so pissed I forgot) bone down for 2 hours, then added some more rub and flipped bone side up for 1 hour. They came out soo good, prob best I've ever made. My point to this is that the temps got away from me and ran too high. So i capped the intakes and exhaust. After 5 min when I reopened them i saw water in the caps and on the pipes. At that point I made up my mind that I will never need a water pan in this smoker. The drum holds moister so well.
So cliff notes: No water pan and add a 4th big arse intake