It starts off as a polymer matrix.
I think I know where you are going with this, especially considering your affinity for the Echols designed stock. Once he designed the stock it is a mere case of replication for each rifle, is it not? However, and maybe I am old fashioned, creating a stock out of wood for a specific client is, imo, part of a true custom. The specifications, checkering pattern, inletting, stock finish and so forth all require skill and patience.
How/why is a synthetic stock custom if you can buy one off the shelf?
Edit - for example, note the woodwork on this rifle:
http://www.hallowellco.com/gary_goudy%20kurz%20springfield.html
or this one:
http://www.hallowellco.com/goudy%20hornet.htm