What I do when I need it wider one direction or the other is add what I guess some folks might call "film strips" row using leftover fabrics. Here's one I made for a niece. This shows it all the way around but you get the idea. Can't seem to find the one I did it only on the sides. Shucky Darn!

You can also change the size of the borders...making them larger where you need them and narrower when you don't need the extra.