Have you considered an inverted conical shape? Not sure how this would affect the airflow but it seems logical that if the heat was in the middle and hit a downward pointing cone and then diffuse up towards the outside. Another thing might be to come up with a design where you can have two overlapping plates that could rotate to close to eliminate temp spikes when the lid is off. Are you using a CNC machine or something similar? I wish I had access to such toys. You can also send any "failed" attempts my way
