Use firebricks to line the bottom of your smoker's firebox. Firebricks are thin, ceramic bricks made for oven use. Purchase enough bricks to cover the bottom of the firebox.
Wrap each brick in aluminum foil. With foil-covered bricks the cooking chamber will take longer to heat up but will retain heat longer. They will also distribute the heat evenly, help dampen temperature spikes and prevent heat from escaping from the bottom of the box.
Place each foil-wrapped brick in the bottom of the firebox until it is lined completely with one layer of bricks.