Over the weekend I put Styrofoam insultation in the lower half of the ceiling and DH and I covered it with OSB as shown in the two photos below.
Before we attached the side wall, we tacked up a layer of aluminum window screen and a layer of 1/2" hardware cloth. By doing it before the side walls were up, the rough edges were completely enclosed between the walls. Below you can see the "view" of the edge of our pool through the newly screened window.
Today we used quarter round to cover up the gaps and seams in the walls. It didn't fit perfectly, especially where the ceiling meets the wall, but it will make the interior look more finished. (Although DH keeps reminding me, "It's just a chicken coop.")
Below you can see how we trimmed out the inside window frame. I was pretty happy about how my mitred corners fit together. I will use wood putty to fill the little gaps before painting.
We also framed in the new doorway/front wall using 2X4s. So we've gotten a lot more done and are able to check a few more things off the to-do list. Hopefully, within the next few days I will get started priming and painting the interior. That will make a big difference in appearance. Our little chicks should arrive in about 2 weeks. What I really need to be doing (in addition to working on the coop) is getting their brooder ready since they won't actually live in the coop until probably late August or early September.