Heating tortoise house

[Nellie Rose]

Anyone have any ideas or tips on heating an outdoor tortoise house through winter?
I have a small Hermanns tortoise I am building a heated greenhouse for, but I also want him to have a smaller to medium size hut to seek shelter in in the winter just in case.
The enclosure is 4 ft by 8 ft, currently no heat for the summer, but my goal is to keep him outside year round (except for nights where he sleep inside, just in case).

He'll come inside at the beginning of winter until I get the parameters right, I have a wifi monitor to keep an eye on temperatures. If they stay steady then he will move outside.

 

[wellington]

A small portable oil filled heater is what mist use to either heat a shed or to heat a night box.
You could heat the main area with the heater and use a pig blanket along with a radiant heat panel for the hide area.

 

[Nellie Rose]

This is his enclosure currently. I have decided that I am going to scrap this one and instead build a custom wooden one with a plastic greenhouse roof and a wooden house I can heat in the winter.
The current enclosure was just a cheap Amazon buy for the summer, but definitely going to try to get the new enclosure done this week and get it in place.