How to fill a hollow object generated from solidify modifier? I have a vertebrae with solidify modifier applied, I need to generate another vertebrae which perfectly fits the inside of the solidified one. I have been trying the Boolean modifier by simply experimenting on a solidified cube, trying to cut a cube of the same dimension using the difference option but no luck.
Unless this misunderstands the question:


Duplicate the solidified vertebra
Apply the Solidify modifier
Select one of the outside faces, Ctrl L select the rest, and delete
Alt N > flip the normals of the inside faces.