hi all Im looking at builder a brick vertical smoker approx 750mm wide, 1800mm high and 800mm deep (external diameters) Was going to line the firebox with firebrick and line the smoker with vermiculite boards. But I’m unsure on the size of the opening above the fire box. Is anything needed between the firebox and the smoker. And does the fire box need a separate exhaust for when you close the vent off on the smoker to regulate heat. Please help.