Tips for making an existing object hollow

Hi Everyone - I see this topic covered often, but it always has to do with creating a hollow object from scratch. What I'm having trouble with is in making an existing object successfully hollow (and for printing, not just for computer viewing). Here's what I'm trying to do...I downloaded a model of a human head from Thingiverse. I'm trying to take off the back half of the head and place an object inside where the brain would be (it's an art thing). Taking off the back half of the head is easy with programs like Meshmixer and the like. But when I go to prep it for printing, it just won't work. I get warnings about things like holes in the mesh or split manifolds, but when I try to auto repair those the head inevitably gets filled in solid and I'm back to square one. I'm sure there must be an easy way to accomplish what I'm trying to do if someone could just get me started in the right direction. Any advice?

Hi there,
Well, you can most definitely start with the most simple trick : you do a 20% reduction of your model, and then do a Boolean operation where you substract the smaller one out of the bigger one.
I never tried it with a mesh though, I would advise to do this on hard models and do the stl creation after…
Hope that helps.

This is fairly easy to do with OpenSCAD, but the STL you start with has to be watertight first, so make sure you run it through (for example) NetFabb's repair first. Here's sample code that imports an STL and slices off first a cube (to slice off the back of the head) then a sphere (to create a hollow). You'll need to play with the coordinates and sizes to make sure it does what you need to with your starting STL, and make sure you save the SCAD file to the same directory as the STL so OpenSCAD knows where to import from.

difference() {
import("HumanHead.stl");
translate(v=[-10,0,0])
cube(20,20,20);
translate(v=[-10,-10,0])
sphere(r=20);
}