The sandbox cover that the manufacturer of our sandbox sells is crap. It's very thin kind of vinyl with a bit of threadlike reinforcement. We've bought a new one every year. (Well, twice now). And every year it just shreds. This year I made one, using the shreds of the old one as a guide. The fabric is from the clearance section of outdoor fabric. It's more fabriclike than the old one - almost a canvas. I used fusible fleece to reinforce the edges that fold over the sides of the sandbox, as well as where the grommets are.
So far, so good. It's not as waterproof as the old one tried to be, but it is as catproof. If I were to make another one, I'd put a grommet in the very center to drain it, so there wouldn't be any puddling. What puddles it's collected haven't lasted long though, so I'm calling it good.