Plastic Fuel Tanks

[DonFromTX]

Need some help, I am a dunce about plastics.
Years ago sometimes we used plastic (PVC, etc?) pipes to make fuel tanks out of and place them in the wing rib lightening holes. I cannot recall exactly WHICH plastic pipes are fuel proof, can someone help?

 

[Auburntsts]

IIRC, the plastic gas cans you can buy almost anywhere are made of High-density polyethylene (HDPE) plastic.