There is a binding tool out that does this in a square edge and it is very easy. I use this often because it closes the entire mitered corner nicely. If you go to
www.animasquilts.com you can see a video demo. It is out of Eureka, MT.
It is called a "Binding Miter Tool". Someone demo'd this at quilt chapter one night and the local quilt shop ordered a bunch for us the next day. I just love it. And highly recommend it. Good luck.