Using "Goop" rather than mole skin or felt in the neck.  Place a paper towel under the Shoulder Plate to catch the drip from the Goop. (click on photo to enlarge)    
Using applicator tip or straight from the tube (as I do) apply a thick bead of Goop at the top of the neck hole  
  The Bead should like similar to this and will quickly run down into the neck hole

Its best to avoid drafts or the having the ceiling fan on, until the Goop has settled.
  If you get the Goop on the outside of the neck area, wait until its dried. Then take a scalpel and trim away. You will also need to reopen the neck hole to accommodate the Shoulder plate Connector by using the scalpel. Let the Goop dry about 24 hours before you attach the head.