This may be coming too late, but I wouldn't cut the piano keys!
I would go back "one more time" to the fabric store, or search through my stash and add another border to the back ... if you are concerned that you won't get both sides "square" and even, then simply add one more width on the back, either side - - you will add "character" to your quilt, and no one but you will know that you had another plan <wave>