Note that I don’t understand the initial post in this thread and as such, this might not be what you are trying to do at all… - I am just answering as a follow-up on what @pascal wrote.
Did you perhaps extract a copy? What did the scene like after you hid the top and outside surface - can you post a screen shot?
Here, with a clipping plane showing the insides, the top surface that you need to extract is in blue, the outer surface that you don’t need is in green.
After hiding those, you are left with the inner surface that you can cap.
… This won’t give you a volume, though. This model is a bit messy and, apparently, Rhino doesn’t like it much. (@pascal, the WIP also doesn’t give a volume on this one - YT issue?).
I extracted the render mesh (after setting it to
Smooth and slower) and ran
Volume on the resulting mesh. This told me that the object is not closed but offered to provide a volume anyway [
Volume = 782777.975 (+/- 0.0001) cubic millimeters].
DupBorder on the mesh shows where it leaks and you can then make a new neck on the original NURBS object by extruding the top surface edges [Red curve in the picture below] a way past the top of the bottle and then trim it with a surface that has an elevation at the same height of the top of the bottle [Blue surface in the picture below]. I then closed the bottle’s inner surface but Rhino still wouldn’t give a volume on that result. Another render mesh extraction gives a closed mesh with a volume of [
Volume = 782774.663 (+/- 0.0001) cubic millimeters].