Polyethylene is naturally a bit cloudy, as far as I know.

Clear vinyl might be the solution. I've bought large, wide chunks from Home Depot by the foot. It's very tough, great for emergency windows that you can actually see through.

10' wide might be harder to find though. As Izzy suggests, a plastic specialist is probably the place to look.

The downside is that vinyl is a toxic environmental pig through its whole life cycle. A shame, because it works so well. So it goes.