OK, I just added the black group at the bottom to do some magic to get the well-formed “Closed Brep”.
(Had to hold my nose and use copy/paste more than I would have liked… 
)
Curve_GM_2023Sep13a.gh (49.8 KB)
There is a “Goal” switch in the black group that optionally gives you the “Inner” surface only.
As usual, there is no single best way to do this… 
