theoretically, what your asking for isn’t implicitly possible
a mesh can be open (e.g. a sphere which is missing a top) in which case there is no volume that can be defined, and therefore no area.
even the smallest rounding error could cause this to happen on an regularly closed object.
there is nothing in the standard rendering pipeline which defines a volume, from which you can select a slice
i would suggest that you use a shader to select a thin slice of the mesh to be rendered. e.g. add the following to the constant shader before the return