I´m sorry, I got the next question :D

Now I tried a similiar solid: I just changed the profile curve more complexe. When I use shell, the surfaces on the bottom and left side are not flat anymore (2nd object). When I use b.diff. with a line(redline), the solid got a strange surface/ mistake on the side( 3rd object).
Is there any way o get a clean result?
Have a nice weekend,
koi