How to build my turtle a home outside? Problem with neighbor's cats?

I have a 13 year old aquatic turtle that has always been kept in a small home-made "pool" inside the house. For years we've been trying to make her a little pond home in the backyard as we have tons of unused space and we know it's sad she spends most of her time in such a relatively small space. Our problem is that our neighbors have 4 big, sometimes agressive cats that spend most afternoons in OUR backyard. They're impossible to get out and we worry they might kill the turtle if we leave her outside unattended. We don't have time to take her outside most days under supervision either. Are there any tips on how to build a cat-proof pond or even on helping with the cat situation?