It is just a horizontal, offset smoker with a propane burner near the bottom of the firebox.
They sell burners for ordinary propane grills, that is what I shopped for. I found one that was oriented in a long, narrow rectangle.
All I had to do was feed it the right amount of gas, which I attempted at my own risk. The bottle and regulator are typical gas grill items, then I plumbed a ball valve inline before the burner, to act as a burner control.
The trick is to feed the burner a proper gas/air mixture. Propane requires a very, very small jet opening in front of the vented inlet of the threaded burner port. The vents allow air in to mix with the fuel jet. That fuel jet was created with my smallest drill bit and some odd piece of threaded brass plumbing fixture, which was the hardest part to locate, for me. Honestly I don't recall how I found the brass plumbing component to drill my fuel jet hole into. I might have soldered it shut first, or brazed it shut, I can't remember. I didn't find it on any shelf though. If I knew what to look for, it probably already exists. Not sure if that helps.
Trial and error results:-
I tried pumping propane straight into the burner at first. All I got was a huge flame thrower, don't do that.
Fair Warning:-
Propane inside a closed firebox is a recipe for disastrous explosions if the conditions are right. Worst case scenario might be if the flame is extinguished, but the fuel keeps pumping. Any ignition source will certainly cause an explosion. The bottom of my firebox is always wide open to dump any unburnt fuel and defeat the enclosed vessel scenario just for that reason. There are no piezo-electric ignitors involved, I use manual bbq lighters
only to ignite the burner.
The ball valve does give good positive control over the flame height though, and thereby control the cook chamber temperatures. I also balance the spread of heat across the full width of the cook chamber with tuning plates under the grill, water pans on top of the tuning plates, and two adjustably damped smoke stacks, one near each end of the cook chamber.
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