Unable to cut mesh with another mesh

Hey everyone, I’m a bit new to Rhino and am trying to hollow out an outer mesh with an inner mesh . My goal is to make a mask of sorts (where the inner surface is flat). However, when I use any command (meshsplit, meshbooleandifference, meshtrim, etc.), either nothing happens or with meshbooleandifference I get a result that keeps the mesh that should have been subtracted with some curves/random remnants of the mesh that is supposed to be hollowed out (third image), which is the opposite of what I want.

Here is the Rhino file:
Mask.3dm (15.7 MB)

I’ve gotten this to work with a few other models, but for some reason can’t get it to work here.
Hence, I’d greatly appreciate any advice provided on how to go about getting trim/split commands to work here.

Hello- your meshes do not fully intersect -

I’d start with that - make sure you have complete intersections. Close the bottom of the head, close off the cut on the outer part.


I didn’t even notice the incomplete intersections. I’ll try that out. Thanks so much Pascal! Very grateful for your help.