Some grades of PVC are UV stabilized--supposedly. I don't know how well this would hold up in Portugal though.
Since you're designing your yurts, you probably aren't afraid of basic carpentry tools or a sewing machine. I have a sketch of a design for fabric gutters--don't know how well they'd actually work. But they sit just below where the roof poles stick out over the lattice/khana. See the sketch.
A yurt is round, yes. But it is only an approximation of circular--a polygon with something like 40-90 sides. Using an octagonal crown ring is doable. You either have to let some roof poles stick out over the lattice more, cut the roof poles to different length (a pain for setup/takedown), or drill your holes into the crown ring to different depths. It depends on your design skills and craftsmanship. Make a good crown ring in any case.