Super top! You can iron in the 45* folds while it is laying nice and flat. Fold the extra border fabric underneath back at the 45* angle so that the 2 folds meet perfectly in the corner. If you are really worried, match up the fold lines, pin and hand stay stitch the seam or use a few drops of Elmer's washable glue in the seams. Lay the quilt back out flat and check the miter. If everything still looks good, machine sew the seams. Check again before cutting away any of the extra border fabric. Proceed to the other 3 corners in the same manner.