What mate to use to seat this fan blade into the cavity?
You shouldn't need mates. Usually, when these are made in a single multibody part, the parts share the same origin. In this case, you didn't send your original part, so you have to use whatever scheme you used in your original part. Molded parts may not have features you can mate back together, which is why we generally depend on the shared origin of the parent part.
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