It's simple "Sweep 1 rail" work, in Rhino. You draw a curve, tweak it's control points, then select "Create circle around curve", check "End" on object snap... Make two similar circles around the endpoints of your curve, and sweep the curve... Just Rhino stuff, I tried to be fairly accurate with this one, but it's not as clean as it could be. Still a lot of loose polys and overlaps, when exported. Nothing visible, really, but I know I could have cut the poly count down a bit...