How can I flip the model to compare it

[Sunderlandkyl (Kyle Sunderland)]

Try using the Dynamic Modeler module:

New module: Dynamic Modeler - parametric surface editing for biomedical applications

We have added a new module called “Dynamic Modeler” to the latest Slicer preview releases (4.11). This module provides an extensible framework for automatic processing of mesh nodes by executing “Tools” on the input to generate output mesh. [3D Slicer: Dynamic Modeler - Parametric Surface Editing for Biomedical Applications] Output of a tool can be used as input in another tool, which allows specification of complex editing operations. This is similar to “parametric editing” in eng…

The start of the video in the post has a section on clipping and mirroring a skull model:

3D Slicer: Dynamic Modeler - Parametric Surface Editing for Biomedical Applications