Hi fordmar,
Thanks for the file. You have shared the final result. The one from the Object Manager image would be needed.
However, The Current State To Object will give you the fastest way if you can find a good quality/speed ratio by using the Cube already in the Volume. With quality, I have the surface during the animation in mind.
Here is a quick solution: it creates two half shapes with the Lathe>Angle or Cylinder>Slice (Perhaps use a Capsule Object>Slice).
This means the one that stays has a static mesh, making texture along the "cut" area easier.
The other half is booled with the animated Cube.
Example:
CV4_2025_drs_24_ANbc_01.c4d
Let me know if there is anything that might not work for you.
Enjoy your weekend