I know what you mean about not wanting to cut a hole in the wall, but I think you'll need to. Montana winters are so cold, you'll need to keep things buttoned up, and a couple of heat lamps probably. But you'll need ventilation in there, or the ammonia will build up and cause respiratory problems. It'll be easier to do it now before the snow flys.
You can cut a hole out on the very top of the building on one side, and another on the top at the alternate side, cover them with hardware cloth. Then you just make a door to cover them. A scrap of wood with a couple of hinges and a hook and eye, and a chain or rope to attach to the hook and eye to adjust how much it stays open will do it. Then you'll be able to experiment with how little you can leave them open to deal with ammonia, without losing too much heat. The rule I go by is that if I can smell ammonia, I need to make a change. And with deep litter, ammonia will build up without ventilation, in my experience.
Basically, draft is when air moves across the birds, and ventilation is when air moves above them, but the birds aren't IN that air flow. Draft is bad, so you can't really just crack a window, because you would have air coming in on them. Ventilation above them moves fresh air in and ammonia air out.
If you need help knowing how to cut through whatever your coop is made out of, let us know what you have and we'll try to help.
Aun
