Need 'winter door' ideas for my coop

[Rhawn]

I never got around to putting a door on my coop (pic below). My very spoiled chicks come and go as they please, I don't even keep the big swinging door closed. Its turning out to be a very cold winter already. Last winter on very cold nights I would nail a board up to cover the large door hole. I need a more permanent solution. I was thinking of some hanging plastic strips like you would see in the back areas of a grocery store. But I'm not sure the girls would go through it. ANYWAY, looking for ideas on door solutions. Something that will allow them to come and go as they please, but still block most of the air. Thanks in advance!

 

[swimmer]

Could you put in some type of sliding door? Either from the side or top? I have a similar set up and have a door that slides in from the side. Sticks out during the day, but keeps the cold out during the night. Once the nicer weather comes back, I just take it off until the next winter. It isn't anything fancy, but a thin piece of wood that slides in and out. Could also use plexi glass or some kind of hard plastic.