I know someone who works somewhat high up at Apple and he told me another reason was that they really wanted to improve the water proofing.

I think you could do this with a pull cut plane after you cut to close to the correct thickness with a hand saw. A pull cut plane will prevent the thin sheet from buckling as you try to thickness it, just have to get clever about anchoring it. You could glue a board to the back and then hook it on a table using that board, plane it until you’re satisfied, then cut the board off.