"How to Keep the Cats Out of Your Sandbox. A natural method for keeping the neighborhood cats out of your sandbox, your garden and your porch. We've been doing this for nearly 3 years - it really works!" lol - she uses human urine once a week!
Mama To Three Chicks: Kiddie Pool Cover - looks easy! I'm thinking to use it as a sandbox cover (made using a kiddie pool similar to this one) To make one that is waterproof, you could start with a really big round vinyl tablecloth.