I see people creating fuel tanks from shaped/welded aluminium sheet, steel sheet, carbon/glass/aramide composite, etc. But I don't see people creating fuel tanks from HD polyethene sheet material. So I wonder: why not? HDPE resists petrol including the ethanol, is easy to shape, not very expensive, a little flexible so it won't dent, and it's possible to weld it using an hot-air plastic welder. Sounds like a suitable material. What's the catch?