Hmmm...that's interesting! I use mine often, and the box temp never gets above around 80! Not trying to ask a stupid question, but are you sure you have the right plate for your model? It should fill the slots, and the whole space from back to door. When I cover mine with the pan of ice, it's cool above after I turn the element off.
By the way - no need to worry about any particular setting on the temp dial. This is a simple "on and off" process, since the thermocouple is isolated from the heating area. It doesn't know anything but the box temp; that's why you turn it on for 20 minutes, and then off.