If you want the corners to end with a single on point square-the same as the rest of the border:
On the sides, you have the correct number of on point squares, but you need to begin & end with a half of a block (HST on point)
On the top & bottom, increase your on point squares by one, and then on the ends, at a right angle to the last square, add the other half square.
If you don't like splitting the square:
Increase the sides by one on point square, and add a small white border to bring the center unit up to the correct width. On the top and bottom, increase by one on point square, but you'll have to reduce the width of the white next to the center.