Creating a plumb wall on a slanted surface

[Claus]

I have created these surfaces, you can see the inner surface is slanted. (Please ignore the Bounding Box Surface)

Now I want to create plumb walls along the vertical surfaces, those are the green lines.

I was wondering on how do I decide which perimeter Curve is normative/guiding in creating the plumb walls. Because sometimes as I’ve drawn with the green, blue and red lines the curves overlap each other both on the floor and as well the ceiling.

My end goal is creating plumb walls as I’ve drawn in the section.

I have no Dynamo script yet because I have no idea yet about the concept on how to do it.
Should I extrude the curves and cut the solids? Or should I do something with the curves?

I am completely stuck. Hope my question is clear.

[jacob.small]

Something to try:

Pull the ceiling and floor surfaces down to a single plane.
Intersect the two surfaces so you have only the overlapping geometry. You may need to thicken them both into solids first.
Get the perimeter curves. You may need to intersect with a plane first if you made a solid.
Create a polycurve from the perimeter curves.
Offset the polycurve by a negative distance to get the inset wall.
Extrude that curve from floor to ceiling.