I have used light bulbs under a stack of supers with a blanket over the top
for insulation but have had trouble with hot spots.

This year I made the happy discovery that the insulated box with a
thermostatically controlled heating cable that I use for warming buckets of
honey and other things is just the right size to sit supers on.  The heat is
well diffused by the time it gets to the supers.  A digital thermometer with
a remote sensor placed in the stack allows me to see what the temperature is
in that position and adjust the thermostat by trial and error.

Chris