I used to use aluminum foil, with firebrick on top of that … But, I found that the foil degrades rather quickly and becomes paper-like.
Just recently, I cut a piece of aluminum sheet (16 gauge, I think) for the top, plus the fire brick. This was my 2nd time using this set-up.
Pretty sure it reached close to 1000℉, but cannot verify … Never seen the needle buried so far ….
And, have never seen the entire bowl get this dark. Usually just the back where the charcoal and wood is concentrated.