We find just white PVC plastic driven into the ground works well.  Usually we can find some scrap PVC from a project, so costs nothing.  One location the vent riser was mown down by township and friendly neighbors several times.  Took the loader to the riser when the ground was damp, and drove a 2" white plastic PVC as deep as it would go.  It stands about 8' above the surface, then I screwed the Higgenbothem riser to the plastic pipe so it would all stay upright.  Nobody has bothered it yet.  If I was very concerned, I would wrap a small piece of conspicuosity tape like on the side of trailers around the pipe in several areas, could help more.  So far I haven't needed to do so, but that would be my next step.

Nice thing about the plastic, if I was the one who stupidly ran over it with a mower, it wouldn't damage my equipment, just have to replace the riser.