Hello
for what it is worth, here a solution with Grasshopper and Dendro, it is surely not the best for the Skull as it is a quite simple geometry, but Dendro puts the power of OpenVDB in Rhinoceros. So it is a good alternative to very good mesh software like Blender, Houdini, Meshmixer …
I also use Clipper for offset
The script has 2 parts
One to extract the upper part and to make the tool to make the upper hole (I offset in 2D using Clipper).
The other part is boolean with mesh using Dendro.
The only trick I learned today is that you need to put bandwidth to 2 in Dendro in order to suppress some mesh problems.
If you want to play, use higher Voxel Size in Dendro, I used 0.15 here.
shell skull.gh (29.3 KB)
The advantage of Dendro is that there are many other possibilities. For example if you want smoother inside. You can double the offset. So it will be more easy to clean …
shell skull2.gh (31.4 KB)