Well, I'm not really sure how much 5" is, but not much there can be done about the width. You can try to just make a deeper seam - and this will work if your yarn is... thin / fine.
One thing you can do is make a crocheted seam - this way if it really stretches more, or shrinks, you will be able to easily correct it. Like my idea here with the flexible width, but your "seam" will be in the inside.
