When we tore down the shed in the back yard, we took down three birdhouses that were hung on it. this one got tossed in a corner and left, until I found it a couple of weeks later. It had square corners when we took it down:
As you see, it now has rounded corners, and to the right is my rounding tool. I have to fine tune it a little so the corners won't be so raggedy in future, but I think I'm onto something.