Hi Francois and welcome.
Having tried PVC I don't recommend it.
Heavy, flexible and sags in warm weather.
It also holds onto heat.
Aluminium is nice if you can get it.
The large size and length required is non-standard.
Any friendly scrap yards near you?
Be prepared to have a thicker center section to join two tubes.
It won't be as light as you hoped.
I used 8" diameter steel ventilation tubing/ducting for my 7" f/12.
Not the usual spiral stuff but smooth and straight with a single, neat seam.
Being galvanized and very thin 0.5-0.7mm it is very stiff and keeps its looks without paint.
Factory and workshop ventilation engineers use it.
Usually where accumulated wood dust would be a fire hazard.
Quite inexpensive and you can usually get it in long lengths of 2m or even 3m.
Each end has a neat, small, rolled flange for added stiffness and joints.
Probably as light and as stiff as necessarily thicker aluminium.
Any furniture factories or ventilation engineers nearby?
A DIY store might be willing to order for you.
They will sell it online here. Highly recommended.
Can you source thin, marine or [better] aircraft birch plywood?
1, 1.5 and 2mm plywood are available.
Boat and canoe builders use it.
I rolled a two layer tube on a baffle former for a 5" f/15.
Wonderfully light and stiff.
It lasted for year out of doors with a good coat of quality wood protective paint.
Fiberglass is usually too heavy to be stiff enough.
It has flexure problems when it isn't.
I tried a gently tapered fiberglass streetlight pole but it was far too heavy.
Carbon fiber is all the rage but expensive if commercially made.
Rolled Phenolic paper tubes have a following.
The problem is weight.
The same goes for cardboard piling tube.
I wouldn't call the cardboard suitable in such long thin tubes.
Then there are square plywood or octagonal tubes.
Or even planks of wood and battens.
They have their following but need some woodworking skill.
They also need protection but are thermally neutral.
If you are stuck in the middle of nowhere I suggest a flexible approach.
Search online and knock on doors of factories and workshops.
I was incredibly lucky to find a small furniture factory which had shrunk.
They had masses of steel tube in many sizes for the price of a couple of beers.
I'd better leave room for somebody else to chip in with their own preferences.
Chris