Closing a STL surface Happy new year!
Problem explanation
I am trying to modify a STL model generated by a CT scan (skin model), in order to close the surface. I am using this in brain simulations and in order to add electrodes on the skin surface, the surface shall be closed, otherwise the Mesh2EEG algorithm does not work. You can check my work here.
Unfortunately I can not share the CAD file here since it is under license agreement from the IT'IS foundation.
I have some screenshots below to give you the context of what I am trying to do.
This screenshot shows the model as is:

This screenshot shows the desired result of a closed base:

Also, ideally I would like to close the holes that are here and there on the surface as you can see from the screenshots.
Tested so far

I have tried to use meshlab with different filters, like close holes, remeshing, fixing, but none worked. I found some tips here and there but none worked.
I created a solid cube, and tried to subtract, but I did not have any luck there

Can you help me on that?
Thank you!
Update 19/01/2021 (Solution)
I finally found a way to this by using the knife tool. After searching I found this video which had exactly what I needed. The info from @Gorgious and @Carlo in the comments helped me get a better direction. In short the process is the following:

Use the knife tool
Create a cut, just above the end, to have different surfaces
Following the instructions of the video, to rip the selection and then separate the meshes, delete the small object at the base of the head.