Firmly press the floor into place and check for stability.  It it rocks, the flange is resting on a slat.  Moved the slat enough to allow full contact between the slats and the sheet metal with no rocking.
When properly set the underside of the assembly should resemble the photo at right.

    Firebricks are placed around the perimeter of the firebox to form the sides of the fire pot.  My layout is two bricks deep with one brick between the sides. The firebricks should be the hard construction variety used to line fireplaces.

     The resulting firepot is approximately 10" square and 2" deep (depending on the size and arrangement of the bricks). This is more than adequate for most demonstrations.