I used the Sweep tool.
The green curves were used for the profiles and the red curves were used for the rails. The blue curve was used for the scaling rail.
If you show points on the curves, it's quite easy to alter the shape as required.

Martin.

Hi wim, thanks for posting the file.

So for the blend method you can set up a cutting curve like this:


Use Trim or Boolean Difference to make an opening:


Select these 2 edges:


Run Construct > Blend to fill in the empty area with a smooth blend surface:




- Michael

Hi wim, this construction method is using Loft style = "Loose" with end points to make a rounded tip directly in the lofted surface:







- Michael