Try a piece of heavy interfacing behind the cubes. not sure how large the cubes are and if it will interfere with the hand of the quilt. if it is ok maybe you can crumple the interfacing to make it softer before applying, just cut it larger than the piece and then trim it out.
Another way to miter corners is to use a half square triangle in the corner and then add pieces to it.
Last edited by j50; 02-02-2018 at 06:48 AM.