My windows are all covered in hardware cloth, then surrounded by white trim on the outside. In the winter I'm going to slide a piece of plex in there and hold it in with thumb turner things--like you get on the back of some picture frames. It won't cut the air completely, but it should cut the majority of the draft issues with little trouble. Then if I want to take a pane out on a beautiful day, I can just pull the glass out again for a few hours, and return it at bedtime. I probably won't do this often because I have a couple of sliding windows I'll probably slide open and closed instead, but the option is there, and it will be simple.
