If you think PVC might be structurally strong enough (it will) but not flexible enough, you can heat it and bend it, curve it and so on. And it is still water proof. I have seen it done for swimming pool installations.
I would start with something as large as 2" near the bottom, and run some feet out to the edges for stability. (The feet do not have to be 2") Then reduce the size going up as the 'wood' branches off.
When you cover that with Great Stuff you can create the look of convoluted wood.