We used one of those portable dog fences to block off our old pool. They are lightweight and easy to fold up and store away. You could place stones (or sandbags) every few feet or so to prevent the fence from being easily moved by the dog and secured the ends of the fence to something.

I will say they aren’t built to stay outside all day, but we had one outside for a couple of years and it was still intact. It was worn down though and the black paint was peeling off. But if you can put it away when it isn’t needed then this shouldn’t be an issue then