Last year I used foam to pad out the shoulders, although this looked okay it made animating the puppet every difficult because of how soft it was. This year I've decided to cast the upper torso in silicon.
The first thing I did was to paint a layer of silicon onto each of the shoulder molds and wrap PTFE tape around the armature joints.
Next I used some foam to pad out the neck and shoulder sections which will allow the armature to move after the silicon is added.
Its very important to seal the foam with a layer of silicon before placing it into the mold. The layer of silicon will stop the foam from absorbing all the silicon in your cast.
Here's what the shoulders looked like directly out of the mold.