Fabric for making sandbox cover?

So what would you use?  We built a rectangular sandbox for the kids today and the old tarp we threw over it tonight has gotta go!!!  I see Joanns online has marine/rv vinyl.  Do you think that would work?  Has anyone worked with that before?  Any tips?

Thanks!!!

I used a heavy canvas. It's lasted over 5 years and still going strong.
The hardest part is finding a way to attach it. Our sandbox is a weird shape and I wanted the kids to be able to remove it and put it back on themselves. We nailed down one side and put a copper pipe in the other end. (I made a casing for the pipe.)
I'm not sure if I would use a tarp since it can rip and fray easily. I would look into getting a heavy-duty canvas.

Brilliant idea about the copper pipe!!!  I was having a really tough time trying to figure that part out.  I'm going to look at canvas tomorrow.  Thanks!