If you take, for example, spineboy's head and delete the internal vertices, you can see the mesh hull has 30 vertices:
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Create a clipping attachment with 30 vertices, in no particular shape:
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Select spineboy's head, select a vertex, ctrl+a
to select all vertices, ctrl+c
to copy vertices, select the clipping attachment, ctrl+v
to paste the vertices
now your clipping attachment fits the head exactly:
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Keep in mind you want to use as few clipping vertices as possible. If you are exporting images/video then it doesn't matter.