BN-LINK Reptile Thermostat Temperature Controller, Digital Heat Mat Thermostat for Seed Starting, Plant Germination, Greenhouse, Incubator, Brooder, Brewing, Reptiles Tank,40-108°F, 1000W, ETL Listed

https://a.co/d/0c14WNoh This is what Im currently using and haven't had any problems and one of them is in a 5"x2"x2"

Put insulation around the box including the doors. Get a thermostat with a plug and mount a space heater on the back - I have a cheap 6" Amazon heater mounted in my cabinet with the thermostat below. Works great but we don't get snow here and whine when it's below 50F. https://www.amazon.com/dp/B01I15S6OM?th=1