…and send me the cartons!
I’ve been negligent in posting primarily because I’m not used to photographing my work in progress. However, I have been working diligently and loving every minute of it.
The egg cartons are needed for the outside of the room boxes. After debating how to “finish” these (bricks, stone, wood?) I have decided to “make” tree bark and attach that as it will give a quick suggestion that the mice live in a tree. This is what the boxes look like, unfinished, inside the cabinet:

Before I made the first batch of bark, I first had to decide how to finish the outside of the windows. Since I had previously inserted them from the inside, the exposed edges were rough and ugly. I turned to my beloved, air-drying Paperclay, a great sculpting material, and applied it around each window. When dry, I painted the clay dark brown.

Then I made my bark. This took some trial and error, but after several sample pieces were made, I used about 10 egg cartons to make one batch. First, I tore up the cartons and soaked them in water. I then puréed them in our blender (don’t tell Andy!) and squeezed out some of the water so I had thick pulp. Then I rolled it out on a towel so it was about 1/4″ thick.
When this was dry (which took almost a week!) I tore it into large pieces and glued them in a staggered manner around each window until I’d covered the side of the box. 
Finally, I painted it with various colors of brown, tan and black and dusted it with green and red pastel chalk.
I’m happy with the way it now looks. However, I have a lot more bark to make and I’m getting pretty sick of eggs–so feel free to drop off any cartons you have on my front porch!