I made a few mods and hit 450 in under 10 min! woo hoo!
Just to bring everyone up to speed. I re-worked my 5′ x 3′ x 3′ oven so it would have 2 3400 watt elements instead of a propane torpedo. It hit 250 in about 10 min and when I shut it down after 30 it only hit 350. The walls are steel studs with cement backer board with a sheet of aluminimum flashing on the inside. The top is insulated with R-19 and has HVAC sheet metal with no backer boad. My ideas to get to 400 are…
(A) Insulate with more R-19 on top of the backer board and get some thicker sheet metal to re-line the inside on top of the AL Flashing. Maybe try to find some thin insulation 1/2 or so to sandwich between the flashing and new top coat of metal.
(B) Turn the panels 180 out and insulate them something better than R-19 and cover with sheet metal. Leave the backer board & flashing in place for a fire safety measure.