Presently I am going to backup the lid on the outside with 1-1/2" of
vermiculite board and also add 1" of ceramic wool to the floor inside,
hoping that the added insulation will slow down the cooling somewhat in
these areas.

One further comment here. Adding insulation to the lid and floor may help,
but never make the mistake of putting extra insulation over any external
metal parts. I have seen the stainless steel casing almost melted and badly
deformed by the heat trapped under added insulation.

Have you tried a kiln vent to even out the temperature, they really do work.

Dave