Honeywell (and probably other manufacturers) do offer a low temperature thermostat for heating that is designed to keep an area just above freezing - goes down to about 35°F. Here is a link if you are interested -
Honeywell Home CT50K Low Temperature Non-Programmable Thermostat for Garage
In my dad's shop he had an old mercury switch style that had a range of about 35° to 55°F which allowed the building to drop down at night as it was not overly well insulated. I used two used regular older style thermostats I got from a furnace person - they were equipped with the mercury style switch in another shop - they were wired in parallel with a switch so that one thermostat could be switched out of the circuit so it did not "call for heat". The other one was readjusted with a tiny wrench I borrowed from the furnace installer so that it would keep the room at about 35°F. Then when I wanted to warm the room up I just turned the switch on which caused the "normal" thermostat to be switched into the circuit and it controlled the temperature and when I was done I would turn the switch off again so the "adjusted" thermostat controlled the temperature.