I would recommend this one.http://www.teltru.com/p-338-barbecue...inch-stem.aspx
Be sure and get the mounting kit as well.
Just drill a hole of the appropriate size (I believe it was 3/4") in the side of the drum and mount it. You'll want the the thermo stem about 1" below the cook grate.
Be aware that in a UDS, a dial thermo mounted on the side is going to read about 40° or more LOWER than the actual cook temp in the center after the drum is warmed up. The difference will be greater as the drum comes up to temp. You'll want to confirm the difference at smoking temps with a digital mounted on the grate. After that, you can go by the dial for smoker temp. For instance, when my dial thermo says 190° the center grate temp is about 240°.
You can also drill out a bolt, mount that in the side of a drum and slide a 12" deep fryer thermo through there if you want to measure center grate temp with the dial thermo. I have some pics of that if you're interested. I did that with mine for a short time but now I just use the short stem Teltru and shoot for 190° on the side.