I have made a lot of piano key borders and love to do them. If I come up short or just a little too long for the sides of the quilt, I just take a small seam in a few keys and it all seems to work out and it is definitely not noticeable. I usually make the corners with a solid piece of fabric or make a pinwheel block and that can be sized very easily. Good luck and I am sure you will be able to fit your border easy.