It turns out my 50 F friend has a Gibson model RT215BCW fridge/freezer, and
Gibson/Electrolux/Frigidaire (800) 599-7569 make a $15 "garage heater kit,"
part number 5303918301 that screws in near the defrost timer in all their
top-freezer products and keeps the fridge working right down to a 20 F room
temp. It uses 120 V. I think it just provides enough constant heat for the
fridge to make sure the compressor runs and the freezer stays frozen.
Less energy-efficient than a freezer thermostat, but simpler.

Some alt.home.repair threads are like ancient Greek "scientists" deciding how
nature works by discussion vs observation. They would sit at a table and ask
"How do porcupines defend themselves?" After enough discussion, they concluded
that porcupines must pull out their quills and throw them at enemies :-)

Nick