I'm sure someone has posted this but maybe someone will find it helpful anyway. We aren't making ours external (because of cold weather) but I found this while doing searches and thought it looked pretty good.
http://www.thegardencoop.com/blog/2011/03/10/external-nest-box-plans/
It looks like the site has more ideas but I haven't explored much of it yet.
http://www.thegardencoop.com/blog/2011/03/10/external-nest-box-plans/
It looks like the site has more ideas but I haven't explored much of it yet.