human head scaled ( turn surface model into solid model)

Original file is from GRABCAD, Credit and honor to following original authors. https://grabcad.com/library/m1-helmet https://grabcad.com/library/scaled-head https://grabcad.com/library/german-helmet-m-40 it's a file of multiple surfaces, . I try to knit all surfaces by adding surfaces to form a closed model as a solid one. Here is what I got,