(GAP/CHINK...don´t know the correct english expression for that...)

Hi, I need your help please.
I tried it myself, but I can't get it right.

In this curved element, I would need a gap that looks like something on the foto.

Do you have any ideas on how to do that?! I couldn't do it with a loft and other concepts. I'm still blind flying MOI. . .
Cheers, MaLa