You might want to look at this thread, which has a lot of interesting discussion on this topic.
I read Forte’s Building a Second Brain a while ago, but I got the impression that the slip box fits best inside the Resources folder, which is what I do. Essentially all of my atomic notes are in Resources, and the other folders are used primarily for projects and other “not knowledge” notes. (Journals and dashboards, for instance)
I also have a “Fleeting” folder in addition to the PARA folders which is where I stage my fleeting notes until I can refactor them.
I’m a big fan of allowing your note structure to evolve organically through links, so MOCs and links are much more important to me than my PARA folders. But I like to have a little more organization in my vault than is typical with Zettelkasten, just as a quality of life thing.