Hi M,
""""""""""super precision is not so important""""""""""""""

The "precision" isn't nessasarily speaking to cutting things at .0001 tolerances.

It is more relating to things like the "Locating pins" you have there. Drilling a hole and sticking a pin in it, is not so easy, when you add "6 pins"

Your mold wont just "slip together". unless you cut slop. Then your mold is sloppy.

To reverse engineer that in CAD, I would just design that block with it's angles, and add the pins. (Leaving the cavity core part as just planes that go across) Pretty simple in MoI.

Then, you can cleanup your poly model by just deleting all the faces that aren't part of your cavity.

You can then import the 2 parts (STL Cavity and NURBS Mold.

Cut the NURBS parts. Then mill the cavity into the surface of that.