I make these nativities from the highest end marine grade solid PVC sheet available. It is made of the same, white material used to for irrigation pipes. Lots of companies make this type of PVC sheet product, mostly for commercial exterior signage and boat parts & finishes. But the material I'm using stands far above the rest. I talked to scientists at every single manufacturer of every PVC sheet on the market and I only found one company with one PVC sheet they would actually recommend (and stand behind) for the abuse a nativity would have to endure.
Most manufacturers told me that though their PVC was marine grade, it could not survive the extreme temperature ranges (particularly the cold) that a nativity would be exposed to. Many were even concerned about the sun turning their material brittle in time. And none of them would be strong enough to hold up in the wind.
The material I use is the most expensive one out there, manufactured by only one company but it is the one the scientists would stand behind for my application, so that is the one I use.
Each year, new products enter the market, but thus far, no material has surpassed the quality of the one I've used for 12 years now. This material is impregnated with special resins and UV filters. It won't turn yellow, ever. It is cold rolled flat. It is more dense than most PVC sheets (thus, heavier than the rest) but it is still a little lighter than plywood. It is naturally white, so there is no need to paint it. It does take paint very well though. It has a real nice matte finish that looks and feels just like professionally painted wood. (In fact, more than once, I’ve gotten an upset email from customers who thought I'd sent them wood, when they had ordered plastic).
If there is a weakness to this material (and it is true of all PVC sheets, which is why I do keep doing the research every year) it does have a brittle point. The material I use is better than most of them, but if a corner is dropped on a hard surface, it will break/chip. So, please treat it with care. If you do, I'm confident it will really last. I expect it to last long enough to become an heirloom.