The EXTG and PROP terrains must have the same overall footprint.
When you have the skirt fitted onto a simplified EXTG group
it should still report as 'solid' in Entity info,
copy that group to one side and edit that copy.
Draw three Guide lines to help locate a new skirt later, on the group's axes.
Delete the top terrain, but keep the vertical skirt and bottom face.
Next copy the PROP group to one side and edit the copy.
This time delete the skirt and bottom face, but keep the terrain geometry.
Exit the edit and select the EXTG-copy group and Edit>Cut it to the clipboard.
Reopen the PROP-copy for editing.
Edit>Paste-in-Place.
The EXTG-copy skirt group in now inside the PROP-copy terrain group.
If the skirt-group doesn't line up with the terrain's axes, then Move it into place
using Shift to restrain the movement in the Red/Green axes only.
The guide's intersection should help with this relocation [if needed].
Explode the skirt-group so that the PROP-copy now contains only geometry.
Erase any Guides left behind too.
The PROP-copy should now contain its terrain top and skirt/bottom copied from EXTG.
Exit the edit and check the PROP-copy in Entity Info - chances are it won't be a 'solid'.
We now need to make it a solid...
Use Thomthom's SolidInspector2 on the PROP-copy to see where the errors are reported.
They will be highlighted in Red.
Fix the ones that it reports as 'fixable'.
If there are other 'non-auto-fixable' errors - e.g. complex holes or inner faces -
then you need to dip in and out of SolidInspector2 and the model itself
[into PROP-copy group > edit] and manually fix these issues in turn.
Use pan/zoom to position yourself over the issues that need fixing...
It's best to have sun/shadows on so you can see holes more easily and the Default-materials showing,
with the back-material being set to a distinctive color - e.g. bright green.
These might be 'flaps' or 'shelves' that need deleting, missing faces that need healing by drawing
over edges to force a face to form, or adding diagonal lines to triangulate geometry and force
facets to form [it's best to triangulate resistant faces to infill facets,
even if the larger hole's perimeter appears 'planar'.]
As you do this you might accidentally form internal 'partition' faces and you'll
need to delete those too using a section plane to look inside the form can help in this regard.
Remember that a manifold solid can only contain edges and faces.
That is no nested groups or components.
Every edge must support exactly two faces.
That means -
No 'faceless' edges [i.e. stray lines].
No edges with only one face [e.g. flaps, shelves or hole-perimeters].
No edges supporting three or more faces [e.g. two 'boxes' sharing a common edge for 4 faces].
Faces should be consistently oriented 'outwards' with the back-face materials 'inside' the form.
Once you have a solid form reported in Entity Info you are good.
Replace PROP with PROP-copy and try CutNFill again on these simplified forms -
the skirts should now match so that hurdle is avoided...
