The floor of the cooking chamber is 1/4" steel plate. The heavy steel plate serves much the same purpose of the fire brick--thermal mass.
The center opening directs hot air & smoke to the middle of the cooking chamber. The round plate sitting on four posts disperses the smoke and heat equally in all directions. It can serve double duty as a convenient place to set a water pan or dutch oven of baked beans.
The 1" raised ring around the center opening is a grease lip to prevent grease from running down into the lower plenum and possibly back into the firebox. The accumulated grease will be drained through a 1-1/4" pipe mounted in the left rear corner of the chamber. The drain pipe is not installed in this mockup picture.
