I use concrete bricks on top of bottom grate. They act as a separator between temperature zones as well as heat sinks/barriers.
If I want 2 zones I put in a wall of bricks (2), If I want a 3 zone, I'll go t-shape with the bricks.
I bank the charcoal on the bricks, not the wall of the kettle. This allows for the circulatory air flow still.