I would look at the kits at cncrouter parts they look good and are aluminum..
just search for them in Google..
and no i do not work for them and i do not own one ...i built one similar on my own with some of there parts..
here is a YouTube video i found..
http://www.youtube.com/watch?v=smu0kA34YsQ&feature=related]CRP4848 Cuts Bird Houses - YouTube