My fire box is 18"x18"x18" 1/4" thick steel.
My exhaust is 4" pipe 24" long. It comes out the end similar to a jambo pit. It has a 5.5"x5.5" square cut in it and a pocket welded to the side that the pipe bolts to. The bottom of that hole is 6.5" from the bottom and the top of the hole is at 12" from the bottom of the cooking chamber and it's center in the pit. My cooking grate is 1.5" up from the bottom of the hole.
The intake vent is a 5" circle. The center of the hole is 6.5" from the bottom of the fire box and in the center of my charcoal grate. That leave 4" under the charcoal grate to the bottom of the fire box. The hole is center on the end in the door.
The fire box to cooking chamber hole is 5"x10". The bottom of the hole is 6" from the charcoal grate and the top of the hole is 1" below my cooking grate.
If anyone has any suggestions or sees anything wrong with my design I would appreciate the help. I am new to smoking and to this forum. I will try to get pictures posted. Thank you.