For my waterproof layer, I use the allergy free, waterproof pillow cases! They are soft on one side and waterproof, although not plastic-ey on the otherside. They are really thin and have worked out great for me!
You did an awesomely fantastic job on this! I know your Mother will love this! I would love this. I also have this book from when I was a child and this picture is one of my top two favorites in the book! Congratulations on such a wonderful job and such fine work!