Thats an interesting solution, especially since you are in upstate NY. This is going to sound like a stupid question, but do you have to heat horse barn? I'm right near Toronto, and I think our winters are milder than yours, and I wouldn't think you have to keep a horse warm during the winter.

Plus, a 4x4 plywood 'window' is not going to get broken by the average horse!

Now 'tall' should the door ways be. I'm guessing 8 feet?