I was going to suggest something like that, but I took another look at the pictures, and noticed this:
the nesting boxes are 2/3's as tall as the door, so if we had the roost as high as the top of the door,
the ventilation would be blowing directly on the hens. :( It was a nice idea.
It looks good! Your coop is very tight, but it still looks tall enough to have the roost at least as tall as the nesting boxes. Raise the roost to nest box level and push it away from the back wall another four inches. That should make them happy. :) Of course, my chickens are always looking for...
I use TSC's blue nipple waterer bucket. I don't know what it is called, but it is 3.5 gal, an blue see-throughish. I hang it above the ground in the coop.
For winter, I'm either planning to buy a heated dog dish, or a fish tank heater to place inside the blue bucket.
As said before...