I've used the craft store foam for years and it does a great job, but the stuff isn't the same springiness as real light seal foam. It's much stiffer and less compactable so it will likely be worse than yarn as far as making the back harder to close. I recently bought some real stuff from Micro Tools and it's quite nice but also pretty pricey. It's perfect for restoring priceless classics or sentimental old favorites.