That's my fault: I only ordered two windows and I had them put one on the front side next to the door, and one on the side next to that. The shed sits fairly close to a fence, so putting a window on that side wouldn't make much sense. So now I have two windows, but they're on adjacent walls and so there's a bit of a ventilation issue.
So I get my stuff together and installed it this weekend!
I bought some trim that kind of matched what TuffShed uses for its window trim. It's got a LOT of texture/wood grain (it may be some kind of engineered material). Basically, I only looked at Home Depot, and just used a redwood board that was only smooth on one side.
I primed the board on Friday after work, so that I could paint it in the morning and cut the hole in the side while it dried. That part of the plan worked rather well. The placement of window is odd, but that's on purpose and luckily the chickens are the only ones who will ever really see it. I put it up as high as I could so that it would be out of the way and in less danger of getting hit by stuff on the workbench. I also mounted it upside down so that the opening was even higher. That way when I put a fan in front of it, it's even further away, and it'll be blowing hotter air out of the shed.
Cutting the hole was messy, but fairly straight forward. It's not perfect -- I really don't like jig saws! Luckily the window and trim and the studs on either side do a great job of hiding my work.
No real issues trimming it out, other than the boards not really sitting flush due to the window trim and the screws in it. But from a few steps away, it looks pretty good.