Hi gunter, sorry I did not see your reply before writing my last post above.

If you've already got the shape you want with your current uncapped version, it's probably easiest to try the "loft between 2 halves" method to build the capping surface.

If you can post your 3DM model file I'll show you how to go about doing that, but the basic steps are to use Edit > Trim to cut the top edge into 2 pieces then do Construct > Loft between those pieces.

The cap surface will be a somewhat more complex surface than one extruded from a 2D curve but it will probably be ok.

In other cases when it's possible it tends to be better to use the 2D cutting curve method I showed above since it results in better quality surface structure which is why I was trying to explain that method.

- Michael