Forgive me if I'm wrong, but does Apple not already offer an option if you want this:
http://www.apple.com/uk/shop/produc...a91c23e71f511c9c951890554c90c93b8b04b2d4ba0d9
http://www.apple.com/uk/shop/produc...a91c23e71f511c9c951890554c90c93b8b04b2d4ba0d9