Btw, the oven floor on a typical gas oven normally reaches at least 700, so, from an oven stress perspective, this setup isn't really pushing the oven much. Instead of just the floor hitting 700ish, your trapping the heat and letting 1/4 of the walls reach that floor temp as well. Because of the heat differential between the top and bottom of the oven, the temps in the top will be almost normal.