Well, I could not give up after I invested so much time in my little project, I decided to try again. The problem with the design was that it simply shed too much heat. I noticed that inside the firebox it was blazing hot, and that the roof of the firebox was warm, yet the cooking chamber would not warm up without an open flame. In my last design, I moved the firebox closer to the main chamber...but this did not work. While it was warmer, it was not warm enough.
After putting in as much time as I did, I decided for one last attempt. This design moves the charcoal box directly under the grate, with a layer of firebricks above to absorb the heat and deflect the smoke...it is like a cross between a reverse flow and an ugly drum smoker. Construction is not yet complete, but here is the design:
The main variable at this point, is how deep the floor under the charcoal basket needs to be. I dug it extra deep then back filled it with loose soil, so I can move it further down if things get too hot.