I call this the AcidDragon Headcast Method... At least I do now. LOL. I'm sure I'm not the only one to have done it this way but after some fiddling, I figured this way out as the easiest way for me to get the peg in and the least waste of resin.
The AcidDragon Headcast MethodFill half your container with clay and stick the head in in the position as shown. Fill it halfway with mold rubber. Allow to fully cure.
Turn upside down and fill other half, encasing the plastic head inside. Allow to fully cure.
Slice into the mold and remove the head. I like to do it down the middle of the back of the head. It leaves a seam line that doesn't even show up most of the time.
Cut a pour channel and an escape channel in the positions shown.
Pour your resin, allow to cure, then pop out the head. Cut the sprues off the chin and neckbase.
Make sure you give some room for the sprues. The diagram didn't illustrate this very well but after some trial and error, I realized I couldn't cheap out on the thickness of the mold or allowing pour channel space because when I removed a head, the wall holding the neckpeg part of the rubber was too thin and it tore the rubber out with the casted head...