I am building a coop right now and am specifically placing the boxes on the INSIDE of the coop for several reasons.
1.) more holes, more joints, more palces to leak air or water and rot.
2.) Build is easier.
3.) Most importantly, it forces you to walk INTO the coop and have a look around when you collect eggs. There are plenty of things that can go wrong in a coop that you might not know about unless you walk in there. You collect eggs once a day and peek inside the coop once a week.....you might miss something you could have fixed if youd seen it earlier.
Thats just my thinking. Of couse the most important reason can be fixed with a little dicipline, but if you are going to go in a look every day anyway, then why not just get the eggs while your in there.