Getting chickens was kind of on the spur of the moment and we didn't design the coop to hold windows but I do have an idea. I was thinking, like you said, of cutting out a window and cover it on the inside with chickenwire or some other wire.
Two thoughts ...
- At the top, so as to prevent weasels/rats/etc. from entering the coop, you could run the gutter guard that looks like miniature expanded metal, so as to fully secure your coop.
- At some lower point, rather than cutting a window, you could install foundation vents, or simply drill a pattern of holes, or cut multiple vents covered by hardware cloth, etc. You could even replace their door w/ a ventilated panel of some type.