I have a long vent like yours on the back of my coops too. The easiest thing to do is to take out the plexiglass and cover the opening with hardware cloth. In really cold weather (we get sub zero temps like you too) I just leave it open an inch or two. With the size of your coop and the number of your chickens that should be sufficient. We also put a moisture barrier and insulation on the ceiling. At Menards we found round vent covers that are about 4" across. If you use a hole saw it is a cinch to put one of those in with a little caulk. put it high on the wall across from the window if you need even more ventilation then just the window.