Customizable Bird House for U-Channel Posts

After seeing Hank Dietz's sign birdhouse (https://www.thingiverse.com/thing:117929), I had this idea to guerrilla style, install birdhouses in my neighborhood. I could install them on the actual U-channel sign posts that are everywhere. Problem was, there wasn't a bird house on Thingiverse that was easy enough to print/install.

So here it is, a customizable bird house, for mounting on (American) U-channel sign posts. It is American, because the holes on American posts are 3/8" diameter, spac