Okay, so you have “Slabs” currently, that are considered floors (Building vision, V-key). I suggest these be renamed “Custom Floors”
Additionally, I propose new “Slabs” be implemented called “Custom Roof” that are considered to be a roof by the game.
Same goes for a custom wall “slab”.
“Free Standing walls” work as intended, I think. Haven’t had any issues using those for what I want.