Air intake adjustment (open)
Air intake adjustment (closed)
Firebox burner
air adjustment
Burner
I built this barbecue pit about 10 or 12 years ago using 10 gauge metal which I had the pieces sheared rather than cutting them with a torch to avoid warpage.
       It has both a 18 x 36 in. grill and an 18 x 36 griddle.  The grill is made out of 1/8 in. expanded metal (two sections) and the griddle out of 5/8" plate steel.  I salvaged to stove burner valves from an old stove and built the burners (2) underneath the 5/8" plate steel.
       The firebox is lined with firebrick and I built a large burner fashioned after a pear burner as the wood starter and heat to use while smoking meat.  The firebox is offset from the grill for indirect heat to make sure no fat drips into the fire that will cause a carcinogen in the smoke coming off of the burning fat.  Instead there is a drip pan that catches the drippings and takes them out of the barbecue pit and into a old paint can hanging on the spout.

       I also have a rotisserie adapted to it that I can cook 2 large turkeys (or the equivalent).  With this set up I can keep the temperature from about 225 up to 375 in the cooking/smoking box.
Drip Pan spout.