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Nothing to add to your setup except maybe some heat shielding down near the burners. I have managed to get a foil-lined tight-box set-up to 140F using heat lamps (for drying dirt samples)...but the burners you have seem that they would be easier to set/maintain a temperature. With the heat lamps we had on/off choices...no in-between.
I use a single burner countertop model and put a small cast iron skillet on it with a big chunk of wood. Plenty of smoke and heat. My brother sent me an old PID he bought from a cheesemaker supply company that just has an outlet on the back that I'm going to try next instead of having to fiddle with the control knob.
I converted two Big Chief smokers similar to what Pete did by installing a couple 1100 watt counter top burners and then plug them each into a PID controller. Very easy to operate. Very hands off.
I have a setup like yours; I raise the heat by raising the burner. Cut a piece of plywood to fit snugly and put it on blocks to reduce the size of the box to the point your heat is right.