[No Pattern] Looking for fabric ideas for sandbox sunsh Sewing Machines Parts and Sales in Los Angeles

As the title says. I’m building a sandbox for my kids, and I’m making a covered-wagon style canopy to cover it. I’m planning on putting in ties so I can raise the sides up, and making it long enough on both ends that I can cinch it closed and keep out the cats while it’s not in use.

I’m familiar with sunbrella type outdoor fabrics, which is my first thought, but unfortunately I live in the PNW where it is drizzling or wet 8-9 months of the year, and I’m worried that the cover would get mildewy in no time. I don’t want to use anything as ugly as a tarp… is there a better option somewhere in between? Or is there something I can treat the sunbrella stuff with to keep mildew away? Would a laminated fabric work (trying to think of the name…)?

Thanks!