Rather then starting from scratch, search for a for a seat file, i'm sure there probably is something in the vectric forum.
http://www.vectric.com/forum/search....&submit=Search

take that file and then all you need to do is add a "nugget pouch" and a "tapered snake rest". I can't tell you how to do it in that pgm but based on the shape in the picture for the nugget pouch it looks to me like you could take 2 teardrop shapes and join them together for the shape giving it a tapered depth from front to back.

then for the snake rest you could do something like a tapered flute. If you want to make it a custom fit I guess you need to sit in a chair and take some measurements for distance and length