Let's say they're plain-woven like sailcloth, but instead of being made from plant fibers like canvas is, they're woven from protein fibers like silk is.  Because it is naturally made and not the work of artificing, there is no 'loose' stannum to draw from it, like there is alumen in alumail.  The fibers come from a flying stannum Caithness called drakeins: winged-snake Caithness that inhabit coastal cliff areas, swooping from a height to spit out a viscous immobilizing spray at their prey.  When harvested and treated, this spray becomes the material that forms ventus sails.