I have been using a New Braunfels offset for about 17 years and I absolutely feel your pain, lol! The COSs are pretty difficult to control strait off of the sales floor but with a couple of mods you can get it under control. One problem with them is that they leak heat due to poorly fitted doors and a bolted on firebox rather than welded and the way I fixed that was to get some high heat silicone caulk which you should be able to find at Home Depot or Lowes and simply laid a bead of it around the firebox lid and the cooking chamber lid as well as the gap between the firebox and the cooking chamber. The next issue was that the heat from the firebox was going strait up the side of the cooking chamber, across the top of the lid and then out the stack which resulted in a 50 to 75 degree difference from the cooking grate to the lid. I remedied that by making a heat deflector or baffle from an old turkey roasting pan. I cut it in half and then not having the tools needed for welding I drilled some holes and attached the roasting pan baffle with screws over the opening of the firebox leading into the cooking chamber. Of all of the mods, that was one of the most important ones because all though it wasn't perfect it made a huge difference in heat distribution. When the heat and smoke leave the firebox it is then directed to the bottom of the smoking chamber and then begins it's rise to the top and to the stack as it should. Some guys have lengthened the stack pipe so that the opening to the stack was just above the cooking grate but I didn't since mine seemed long enough already. Finally I did make a simple charcoal basket with expanded steel and was able to get a longer, more controlled burn but I eventually just went with a small wood fire which I start with a chimney of charcoal and a couple of small splits. Once I get the blue smoke going I just add a split at a time every 45 minutes or so and the old offset maintains heat pretty well with that. Just maintain a small clean fire, keep your exhaust damper completely open and let your intake damper do the work.
