Weedweave
This is the High Fantasy equivalent of plastic; its name is fairly literal, as the first step in the weedweaving process does involve weaving together plant stalks into a hollow structure of the desired shape. The mage then imbues the structure with magical energy, which permanently transforms it into something reasonably light, flexible, waterproof, fireproof, and non-conductive. Assume that a weedweave item is about as strong as hard plastic, and that a weedweave garment is about as supple as thick cloth.