I would use a very low durometer (soft) silicone for the job. Dragon Skin 10 or Ecoflex both come to mind. Both of these are very soft and very stretchable. I think I would just cast the entire head and then grind or remove what you don't want or need on the cast.
To cast, you can use Smooth-Cast 300 for a clean white casting.
Of course, there is a bunch of more detailed things you need to do and consider. I think you can use the search function for that information.
You can find details on the materials listed and can order them from here
www.tfbplastics.com. Look for the tech sheets on each.
Regards,
Doug