Hi gunter, so the sort of most common way you'd usually go about it would be something like this where you'd initially build a larger solid with planar ends on it like this:




Then rather than trying to construct things directly to the wavy top, you'd form the wavy top by drawing in a side profile curve like this (it usually tends to be good to use 2D curves as much as possible rather than doing things with all wavy 3D curves):



Then select the main solid and run Construct > Boolean > Difference, and use the 2D curve as the cutting object. That will cut the solid into 2 pieces like this:



Then you delete the piece you don't want and you'll be left with the result like this:




Now the way the cap works in this case is it comes from a surface that is extruded out from the 2D curve and then cut by the main solid. So if you turn on control points of the cap surface you'll see it has control points like this:




And if you remove the trim curves you can see the full surface for the cap underneath the trimmed away areas looks like this - it's an extrusion of the 2D curve:




That's basically the usual way you'd want to do it. In your particular case here you may have to do something a bit different because of how your object is shaped with your one side zooming off at a steep angle and the wavy cut kind of waving above what would normally be the top plane of the object. So you may need to do a somewhat customized version of this type of thing by building an extended surface that does not actually have a planar end on it and then using Edit > Trim to cut that extended surface with a side extrusion. I'll try to make an example of that next.

But I hope this example here shows the sort of main way you want to get those types of caps in place, they usually should come from an extruded surface that has stuff cut away from it.

- Michael