There's a super glue used with PVC "lumber" called "Super" or "Red Hot" I don;t recall exactly. We used it a lot in exterior trim work with Azek and Koma board.
Another item of possible interest is that the two above can be heated and curved in either direction. It takes specific and even heat, along with a form to mold it around; but it is done all the time to mfgr arched trim.
I'm considering building a ventilator or hatch for the head out of it, so I'll watch this thread with interest