I've tried a few more techniques that are even more simple. I've used fusible to iron down the miter, sew it, then trim off the fusible.
Sharon Schambers has videos about using Elmer's School Glue on lots of things and I'm going to try that next. It's the same principle of how she does a 45 degree join on binding. Fold the boder under and iron the miter, put a very small line of glue on the fold, lay it down and press to set the glue. Sew on the pressed line.