I have cooked a pig ...... 2' x 4' expanded steel rack... 3x5 would be good..... fire about 36" below the pig.... an aluminum cover to hold in the heat....
Mild steel sides to the fire pit and fire brick lined firebox to hold heat..... a few air inlets to control the fire... a few air outlets to let steam and smoke out at food level in the top....
I made this about 25 years ago.... the steel plates in the side of the fire pit are scrap small pieces to allow for expansion and avoid buckling and warping.... just tack welded in very few spots......
......Roll out firebox for easy adding of fuel...
...Supports for the spit that is turned by a 2 RPM motor......
...Where the spit enters the Cook Chamber and where the motor sits.....
I have cooked everything imaginable on this.... Brisket, Turkeys, Salmon, Ribs, 40 chicken quarters.....
The fire doesn't need to be big.... Temps are usually 200-225......
You will need a drip pan under the meat rack that drains out of the smoker.... If fat drips on the fire, you will have an "out of control" fire that burns up everything..... The pan needs to be as wide and long as the firebox.... fat dripping down the steel sides of the fire pit, usually aren't a problem.....
The design above came after about 15 models that needed tweaking... this works well for me but a few improvements are always possible and probable.... I keep tweaking all my smokers trying to improve on their design..... I don't think there is a "perfect" smoker.... I ain't found it.... I've probably built at least 50 and still working on them....