What type of plastic is best for outdoor usage? Like resistant to constant sunlight and water exposure and strong enough. It will be used to encolse part of (small size bird) aviary. Also please specify how much thickness will be enough for it.
Corrugated roofing panels, either clear polycarbonate or fiberglass skylight panels, should do the job. I'd put more trust in the light/weather resistance of fiberglass than polycarbonate, but both are readily available.