"Bruce L. Bergman wrote
...
PEX is possible.

The special Air rated PVC is made for piping up chemical refineries
where regular steel pipe would dissolve in weeks and they have to use
Plastic and Fiberglas and Stainless to build the whole place. And
AIUI it's HELLACIOUS EXPENSIVE. Not for the price averse.
-- Bruce --

Another one for comment:

I've been using grey plastic electrical conduit for projects. It's cheap,
machines nicely and the thick wall permits strong threads. Is it any good
for pressure?

jsw