Hi Koi, so then to put on the top as a separate piece you do something like this - I made a taller version of the one rail sweep to have more material to work with, it looks a bit weird at top initially but it's ok that doesn't have any actual bunching or self intersections and it will be getting sliced off:



Set up a cross section and rail for the top cap something like this:



Sweep to make the top cap base surface, note that you want this to be a simple extended 4 sided surface so it can be a relaxed surface that you will trim instead of trying to force the initial surface shape directly to the ending boundary edge:



Offset cap surface since you're making a thin wall shape:



Trim surfaces with each other and join:



Apply a G2 fillet:



- Michael

Basically on your version trying to keep the top clamped together while also having the bottom rotate is just too many different shaping forces all trying to be applied at the same time.

2 rail sweep between rails of different lengths can also just generally apply a type of shearing type of effect as it is sliding things along the rails at different speeds and things can be prone to getting bunched up if they are also trying to do other kinds of shape transition at the same time.

- Michael