Instead of using FreeCAD, I would import the mesh data into a vertex-based 3D modeling software, such as blender.
After stitching the model in any areas where you still have gaps, you remove any vertex belonging to areas you do not want to use for the mask. This leaves you with pretty much a skin-tight base for your mask. Select all the remaining surfaces ...
It depends on the software you're using, but here is an example with TinkerCad.
Step 1: Import your 3D model (imagine that the ball is a head):
Step 2: Change its type from "Solid" to "Hole"
Step 3: Create a "Solid" box around your imported model.
Here is the inverted Solid/Hole version:
Here is the Solid version with the ...