Hi Colin,

I took a stab at your model.. I drew a planar curve approximately following your
3d curve. it consists of an arc, tracing the center of your curve and a freeform curve (through points) for the side section,
snapping to the end of the arc and then placing the second point of the freeform curve so that it lined up tangent to the arc.
That relaxed the bend where the band was crimping up and breaking.
It's off by less than 1/10 of a mm from your original curve.

Then I drew a circle for the finger hole, extruded a cylinder and trimmed the cylinder with the planar "profile".
I copied, pasted and joined the edge curves of the trimmed edge of the cylinder,
and then trimmed the copied curves at the 3 and 9 O'clock positions
of the ring. (quad points). I also trimmed the circle at 3 and 9, removing the top.

From there it's just one rail sweeps for each the top and bottom parts,
without capping the ends. Join both sections and you're done! :-)

Jesse