I can't find a mate that works to attach the straps.
Start a 3D sketch and place points on the inner surface of the helmet and mate to the points.
Where are your planes in the Helmet. Also the ends of the straps you could put point but the vertex's at those point would work just fine. Where are your planes in the straps also.
When building things it good to watch how they will connect and add planes or Axis to help later on.
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