On the small side of my coop I used an 8x10 picture frame with plexiglass. I cut a hole to fit the frame. Covered the hole with hardware cloth and used hinges on the top of the frame. I just use a piece of wood to hold the window open and have a lock on the bottom to lock it up at night. It sits flush with the coop wall so no drafts. Works for me.
Hope this works

Hope this works