Have you tried passive solar type ideas?? Barrels of water warmed during the day that hold the heat thru the night? Same with rocks or concrete blocks? It would also depend on your area temps.
There is or was a YT channel called Swedish Homestead (I think that's what it was) that raised chickens inside their polytunnel type greenhouse. It's been years since I watched it, but IIRC they had done that just to keep the ground turned and fertilized, then would move the chickens to let it compost. Not sure if it was part of their video, but I would think the heat generated both from the chickens and then the hot compost would help keep the greenhouse warm......even in Sweden.
OK did a search and found they changed their channel name. I think this is the right video, maybe not.
I had bought a lemon and a lime tree this year. They are sitting outside, right next to the house on the deck facing east, for the same reasons as you mentioned......no room in the house. So they do have some protection and so far we've had a mild winter here in PNW. I am hoping they'll survive so I can plant them outside and this is my way of getting them used to it. So far, so good