Since you've already assembled them, I'd miter. I glue baste mine so they don't shift at all. What I do is lay one edge down, cross the other over it, fold it under so you get a 45 degree angle, press, align and glue. Unfold it, you can also pin in place if you want that extra security, sew on the fold line.
Alternatively, you can even add a cornerstone if you decide not to miter.