How to fix a self-made sidebar?

Dear Team,
I installed the "Fixed Widget and Sticky Elements for WordPress"-Plugin.
In Elements I built a Block-Element as Right Sidebar.
If I go to Appearance -> Widgets -> Right Sidebar is empty, therefore I cannot use the "Fixed Widget Plugin" to fix my new-built sidebar. I also tried to set it as a Hook "before-right-sidebar-content" - the widget's field remains empty.
If I try to fix the block-element via css with position: fixed; right: 0%; top: 30%; during scrolling it hides behind site content - so this is not a good solution for me.
Question:
How can I build the right sidebar and fix it?
Thank you!