Is there an alternative to extracting subtools from a mask?
Yes! Another alternative to extracting subtools, and one that provides a great deal of control over both your extracted mesh and its thickness, is the retopology tool. You can easily retopologize only part of a mesh, paying careful attention to your edge flow (if this is important to you), and then append the retopologized piece as a subtool (also controlling its thickness as well) to your original model.
