A styrofoam head would be the best bet for something like this as it's so much smaller than a human head. Those busts are used to make masks that would fit OVER a persons head so You'd want something smaller to sculpt something that's directly 1:1 like a bust.
Normally, you would just take a piece of plumbers pipe or something screwed into a board and then bulk up a small orb/globe/head shape in tin foil and then sculpt onto that.