You hit on some important points in your post. You said that they must have heat when enclosed in a small area without much space to run around. Coop size is important for those of us that live in cold climates. The coop must be big enough that if the chickens choose to stay in there all day they have enough space to walk around and scratch about and be active and doing normal chicken things. Activity improves circulation and helps keep them warm (just like when we exercise we begin to feel hot, even work up a sweat). Encouraging activity on cold days is good too, a bit of scattered scratch that they need to walk around and hunt for will do the trick. But
@DobieLover has a good point, they aren't mammals, they have a higher body temperature than us and feathers designed to regulate their body temperature. Also many "coop heaters" are not really coop heaters, but create a warmer area where chickens can huddle or a panel they can snuggle against, they aren't designed to heat all the air in the coop and raise the overall temperature in the coop.
Some chickens tolerate extreme cold better than others, and some chickens tolerate it longer than others. Extreme cold for a day or two is one thing, but a week or two is something else, over time it may wear on your chickens. It's important to check on them often when it's cold and their body language will tell you if they are doing ok or not. A chicken that is lethargic, standing still, huddled/puffed up, standing on one leg, shivering, eyes closed, is a chicken that is struggling with the cold. A chicken that is walking around, alert, active, scratching, doing normal chicken things, is doing ok.
I'm sorry I cannot specifically recommend a heater, I generally choose not to heat my coop. I have used a heat lamp with a red bulb designed for poultry in the past for some particularly young/vulnerable pullets that were not hardened to the cold. The key is not only to have it secure, but to have it positioned in a way that they can be under it or move away from it, and where they can't easily fly into it and potentially break it.
Keep an eye on them, and hopefully they all get through it just fine.