I just finished this quilt, and I made mine larger by adding borders. It is still square though. It ended up being 61 inches square. I added a two inch inner border and a five inch outer border.
I know that isn't what you are wanting to do, but perhaps it is a starting spot. If you made the top and bottom borders bigger...well, now I can't visualize that either, but...there ought to be a way to do it.
I will attach a picture of mine. It isn't quilted yet in the picture, but I haven't taken a picture of the finished product yet. I literally just finished putting the binding on it. Perhaps the picture will generate an idea for you that will help...
Dina