Years ago, I used to sculpt life sized objects. I also have an industrial design background from my school years.
I also use 2D profile photos, steels and tools of the trade to cut and shape. Similar to how auto designers build up their protodesigns.
You can now use laser scanners ($$$) and cnc to produce these body plugs, but then that takes away from the joy of the process.
renshape
http://www.one18th.com/GetGallery9970.htm
http://www.one18th.com/GetGallery10156.htm
http://www.one18th.com/GetGallery8561.htm
clay
http://www.one18th.com/GetGallery8999.htm