I have an idea. You'd probably have to make this yourself, since I don't think you can find it, but here's a thought.
Have you seen a louvered attic vent "window"? They allow air to pass through, but by necessity must be made not to allow water in. Look them up, they're easy to find, but being made of solid materials (wood and aluminum usually) don't let a huge amount of light in.
Consider making one of those from plexiglass. Rough idea here, and keeping in mind I've never actually made one of these: Make a frame for the whole mess to sit in, in the size/shape of your desired "window". Attach small pieces of wood along the inner edge of your frame at the angle you want your louvers to sit at, measured so that there's about 2-3 inches of space between each louver to allow air flow. Make sure there is overlap so that water dripping from the bottom of one louver will drip onto the middle of the louver below that. You will want a pretty steep angle, and always angled towards the ground. Probably 45 degrees would do it.
Once you have the whole thing done--being sure that the bottom louver will overlap the top edge of the bottom side of your frame by about an inch (this keeps water from dripping onto the flat surface of your frame and dripping into the coop, and also prevents wood rot in the long run) I recommend putting 2 coats of a high gloss exterior paint on it BEFORE putting your louvers in. This protects the wood.
That done, cut your sheet of plexiglass into appropriately sized louvers, and set them on top of your "ledges". This is probably best done with the frame laying on the ground. Use either caulk or liquid nails to secure the plexiglass louvers to the wood. I would recommend following up with attaching more pieces of wood to the top exposed side of each louver as you go, nailing or screwing those in so that if your caulk/liquid nails ever fails, the plexiglass cannot go anywhere.
Once all those are done and your adhesive is dry, flip it over so that the inside of your window is exposed, and staple a piece of heavy duty screen mesh into place on the inside to limit insect access to your coop, and prevent any mice or snakes from getting in.
Install your window into an appropriately cut hole in the coop. Make sure to caulk around the whole edge of it, and consider putting a frame in to beautify the edges--or composite quarter-round meant for outdoor use would work equally well.
You could apply some peel and stick window tint if heat is an issue in your area, and you want some light in the coop but not too much heat, before you put the plexiglass in place.
Not sure this will help you or not--it depends on how handy you are I guess--but it's what I'd do in your case if you want a window that's both waterproof and lets light in, if aesthetics are important to you.
