On low spots and places where I sculpted too aggressively, I've begun filling in with a mixture of white glue and sawdust. The styrofoam helmet is still four separate pieces -- I want to do as much sculpting as possible on parts before I glue together the whole thing. There will still be sculpting to do after it's all together
If you look very carefully, you can see that I'm wearing a cloth helmet with odd ridges over the forehead, similar to what the pilots in Star Wars wore. Since there's so little reference material, I took some artistic license.