We built an exterior box similar to the link posted up above. DH made the opening a little bit too big so we have two spaces on both ends that will serve well as insulation. We walled them off (about 3" wide) and will stuff some of the pink styrofoam in them. As for weatherproofing, we caulked well all over and I'll cover the caulking with a couple of coats of paint. If we notice any weather getting in I'll add more caulking.
The main reasons we made ours exterior is to free up space. It would have had outside access even if they weren't sticking out of the coop. The reason for that is my kids want to collect the eggs but don't want to go in the coop. I suspect they'll lose interest in it quickly but it will be nice for me not to have to put on my boots every time there's an egg in a nest. I will still go in the run to give them treats every day and the coop to check their food and water. Plus our coop will have windows all around so I can easily peek in any time of the day to see how things are in there.
The main reasons we made ours exterior is to free up space. It would have had outside access even if they weren't sticking out of the coop. The reason for that is my kids want to collect the eggs but don't want to go in the coop. I suspect they'll lose interest in it quickly but it will be nice for me not to have to put on my boots every time there's an egg in a nest. I will still go in the run to give them treats every day and the coop to check their food and water. Plus our coop will have windows all around so I can easily peek in any time of the day to see how things are in there.