I am converting my 12x16' green house into a large cold frame. I don't want the temp to drop much lower than 30*. I have a 240 volt, potable heater but the temp only lest it go as low as 45*. Is there a way I can install a Thermostat switch externally to only let the heater kick on at a lower temp?

Like this?

https://www.amazon.com/s/?ie=UTF8&keywords=portable+heater+thermostat&tag=mh0b-20&index=aps&hvadid=78271538937790&hvqmt=b&hvbmt=bb&hvdev=c&ref=pd_sl_20zsd2z64j_b

https://images-na.ssl-images-amazon.com/images/I/414oSDTO00L._AC_US327_QL65_.jpg (https://www.amazon.com/Houzetek-Programmable-Thermostats-Multifunctional-Temperature/dp/B075L9HJDK/ref=sr_1_10?ie=UTF8&qid=1534427501&sr=8-10&keywords=portable+heater+thermostat)

Or simply spring for a heater with a programmable thermostat built in.

Search "240VAC line thermostat". Lots of options