Two reasons that I can think of exclude PVC as a desirable material. The first and probably most important is the heat problem. You will be terribly plagued with tube currents.
Secondly I would thing that rigidity would be a problem that would be exaserbated by the heat problem. Doesn't take a lot of "sag" to really mess up collimation! Related to this problem would be vibration damping.
IMHO, I wouldn't consider PVC.
Mike
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