The firebox is directly under the tank. There will be a plate that lines the bottom of the tank and separates the firebox from the cooking chamber. The plate will have openings that can let the smoke into the cooking chamber, or the openings can be blocked off to prevent smoke from entering the cooking chamber.

Note the dashed lines chalked inside the tank. The upper line was marked using a level. It is the same height as the bottom edge of the door opening (be sure to level tank left/right and front/back before drawing these marks!). The lower line is marked by measuring down from the top line. This is where the plate will be installed. Removable racks will have short legs that sit on the plate.
Skids will be installed on the bottom of the firebox. I opted not to put this on a trailer at the advice of Dad. He said it makes it nearly impossible to steal and prevents people from asking to borrow it.

The slope on the back makes the wood in the back roll down as fire burns. I still need to add the firebox back panel and door

Here I'm welding on the bypass chimney. There will be a T-joint here with a removable cap on the horizontal pipe for cleanout.
There will be two chimneys. One is for smoking and one is for 'bypass'. The bypass chimney will pull smoke from under the plate. In this way the smoke will never touch the meat. This lets you cook for many hours without getting the meat too smokey tasting or having to wrap the meat in foil. The smoking chimney will let smoke go through openings in the plate. The smoke will pass over the meat and out the smoking chimney in this way.
I still need to fab the ash door to cover this opening at the very bottom.

This is the air intake with a sliding damper. It's on the back/under side of the firebox

This is the handle that will be used to control the air damper. There are notches for setting at 1/8th increments from closed to wide open. Squeezing the handle pulls the pin out of the notch. The quadrant will be welded to the cooker, and the handle will have a linkage rod that pushes and pulls the sliding damper. I cut this using a circle cutter attachment I made for the plasma cutter. The notches I layed out with a protractor, made a punch mark, drilled out with a drill bit, then cut the side out and squared up the hole with a cutoff wheel.

This is the view down into the firebox. I used catwalk material for the grate just because I had it laying around. This will probably burn out. I'll replace it with the same material or possibly something heavier depending on how long this lasts.

Here is the firebox lid. The handle is on the end on the lid and not in the middle since flames will be coming out of this opening when you load more wood. The handle also has a downward bend to give your knuckles clearance from the tank.

Here are the dampers on the chimneys. These are either opened or closed. There is no middle setting since the airflow is controlled by the sliding damper on the incoming side. The knot in the shaft is a screw adjustment to make sure the dampers close down tight

Here is the damper control handle. It's just an eccentric on a shaft. It breaks "over center" and locks in place in both the open and closed positions.

Here are the tubes and dampers for controlling smoke contact with the meat. This control breaks "over center" and stays in the open position. Gravity keeps it in the closed position. These tube connect the firebox (under the plate) to the cooking chamber

This is the plate that seperates the firebox and cooking chamber. The plate has a slope of about two inches toward the far end. The plate is also progressively 'cupped'. The 2" angle iron down the middle helps give a relief for heat expansion and gives a trough to let the last little bit of water drain. The small amount of daylight at the end is the drain valve. The plate was welded in seperate pieces since I don't have a heavy press to bend the compound curve. Instead the compound curve is approximated by changing the tilt of each plate before welding. The racks that hold the meat will sit on top of this plate. Notice there is no slope on the plate where it hits the wall of the tank. This gives a flat place for the legs of the rack to sit.

This is a cleanout hole for cleaning out underneath the plate. The place where ash can be trapped is small and can be reached from inside the firebox. I would not include this cleanout hole if I was building this cooker again.

This chimney vents the cooking chamber.

The water drain. I use a ball valve to avoid getting the valve clogged with bits of fat.

Here is the ash cleanout door....and in the open position.

This is the 'toggle' hinge on the ash door. This lets the door swing open then drop down and lock in the open position. The other end of the ash door has the mirrored twin to this hinge. The hinge linkages are made from 1/8" flat stock. I would probably use 1/4" flat stock next time to give more sideways stiffness to the door.

Here is the completed cooker on the burnout run. I built a good fire in it and got everything up to temperature. This helps burn off the residual propane smell.
The damper on the right chimney is open, but the left chimney damper is closed. This means no smoke is going through the cooking chamber.

Even with no smoke going through the cooking chamber it is still hot enough for cooking. The water will prevent the temperature ever climbing over 212F.

Here are the racks. There are three racks. Each one is narrow enough to be removed through the door. The rack on the other end had to be cut a little in the corners to match the curve of the tank. The curved portion on this end of the tank is used by the smoke controls. Notice the water dripping down the back wall and the temperature gauge showing around 180F. There is water under the rack (can't really see it) and steam is filling the cooking chamber. The steam is condensing on the upper side of the cooker and is dripping back down. You don't really see much steam in the picture. Most of it blew away when I opened the door. Cook for 18 to 20 hours with temperature between 180F and 200F with moist steam... guaranteed to give you tender juicy meat. The last few hours you open the smoke controls, close the 'cooking' chimney damper, open the 'smoking' chimney damper, and smoke using a low fire that has been burned down to coals. Don't stir the fire or add more wood while smoking. Stirring the fire may blow ashes on the meat, and smoke from fresh wood doesn't have good taste.

Here is the contraption I made to hold the doors open. I didn't want a huge heavy counterweight. The doors are heavy enough that they would need a heavy counterweight if I used only a simple balance. These "lollypop" counterweights have a break-over effect. When the door is closed the lollypop is mostly upright, and there's leverage that makes it easy to move the lollypop. When the door it open, the lollypop falls behind it's pivot point. There almost no leverage and it's very hard to move the lollypop. The bolt and jambnut let you set the height that the lollypop cannot go below. Set this too low and when you pull the door to close it, the lollypop will jamb against the cooker. Set this too high, the doors are too easy to close, and you may get knocked in the head if the wind blows the door shut.

Fire in the hole!

I need to do a little more work. The ash can door doesn't really have tight enough tolerance to stop airflow when closed. The result is the fire burns too hot even with the intake damper totally closed. When cooking this isn't really much of a problem. A fire that's too hot just makes the water boil faster. The temperature still stays below 212F. You do have to be careful that you don't boil all the water way, then the too hot fire will burn the meat. When smoking there is still water in the chamber, but the smoke coming straight from the fire isn't cooled by the water and can burn the meat. As a result smoking will be too hot until I seal the ash door better. Nothing a little welding and grinding can't fix.

These is the CAD file I used when designing the cooker. I use qcad on linux. The DXF format may work with other CAD programs. cooker.dxf
Here is the CAD file for the toggle hinge. linkage.dxf

Here's the tool I used to design the cooker