First experiment was not a success. My heat source was a small camp stove burner that screws on to one of the small, 2 pound Coleman propane bottles. I put a 30 gallon drum lid on top for a heat baffle and to catch grease drippings from the sausage. This dinky burner was able to get the temp up to 170 when it was 70 degrees plus outside. But when it was below 50 with some breeze, and summer sausage inside, it maxed out at 120. No joy there.
To save the day, I ended up using a small supplemental basket of lump charcoal, feeding it 2 to 3 medium sized lumps per hour. Remove the lid and all the heat escaped. Once the charcoal kicked in, temps would spike to 180 plus, then ease on down. Temps yo-yo'd all day long. Took the better part of 8 hours and 2 full gas bottles, plus charcoal, but I got it done. Had some grease drip out the sausage, so at times, I was on the edge of too hot, but still came out with something edible.
Bottom line is gas works, but I'm going to need a bigger burner. This was not on my UDS, but with a different barrel set up to be a hot smoker. It has a large hole cut in the bottom for the gas burner. Next try will be with a larger gas cooker......a Bayou Classic.
Since then, I've found another burner assembly you can buy:http://www.tejassmokers.com/images/HPGX2.JPG
This is the burner and air intake as two pieces. You install your own black cast iron pipe in the middle. Any length. I could see that pipe extending through the side of the UDS through one of the vent holes, so the burner was on the inside, air intake, etc. on the outside. Bolt the burner to a piece of 4 inch channel iron set on the bottom of your UDS. Set a steel plate above it to act as a baffle (set on fire bricks) for even heat distribution, with wood chips on that plate.
Such a burner arrangement would allow you to set a precise temp and hold it for hours if need be. About the only modification needed for a UDS would be one slightly oversized vent hole for the burner pipe and a place to hang smoke sticks. Pretty minimal mods to make.