The corrugated has worked well for me. No issues with water, and it has held up well to sunlight UV over the 3 year registration periods. Corrugated is lightweight and flexible compared to most of the alternatives. I fold it in the middle along the grain for storage/transport and attach to the fabric with bungee cords under tension to keep it properly deployed for visibility.
There are lots of different sheet plastics, if you can find some that will work. But that white corrugated plastic board used for signs also works. And if you can find some white PVC baseboard at Lowes it will work too. Heavier, but workable.