Couple of thoughts if you still want to go indirect/offset (looks like ThermalMass's build is not an offset). If I understand your original description, your firebox will be concrete (cinder) block with a fire brick liner. You could just stop the firebrick liner one course (about 2-1/4 inches short of the top of the outside layer of block, cut a (i.e. 1/2 inch) steel plate to fit tightly and cover with refractory mortar. You might consider extending the steel plate into the cook chamber a few inches to act as kind of a deflector plate. Based on ThermalMass's experience with his WFO, I'm pretty confident this kind of setup would get you plenty of heat & smoke. Final thought, you will probably want to control your fire at the intake side, so some kind of firebox door with adjustable air intakes should be part of your plan. Good luck, this sounds like an exciting project.
Originally Posted by Just BS
I haven't figured out how to enclose it yet, but that is the plan. I am leaning towards pouring refractory concrete into a mold. Still working out the details.