You put them inside scrollFix and fauxBody, when they belong after. In fact that's the entire reason .scrollFix exists. A sibling level container can have it's scrollbars overlapped and/or ignored.
Though honestly trying to integrate this type of modal system into the shit-show that is turdpress is likely an effort in futility unless you throw 90%+ of how it wants you to build things in the trash... something that led a friend of mine to an early grave AND resulted in neutering the upgrade path and compatibility with plugins.
Because it's that shite.
Shite like I had to use the document inspector because all that "JS for nothing" in the BODY made it impossible to even find the content markup.