I'd recommend Michael Yardley's book Gunfitting; the quest for perfection http://www.amazon.com/Gunfitting-Quest-P...TF8&s=books

It's a little 100 page book with invaluable points.

I'd also recommend you take an existing stock and modify it by glueing blocks of wood, Bondo'ing, or a combination of both. Then shoot the gun for a while and modify as needed. Once this is done and a shape/dimensions finalized, the "pattern" can be sent to a maker with a duplicating machine. This will save countless hours and possibly save you from messing up a good blank.