exactly... you dont want ANY sort of leway. If it flexes under your thumb pressure imagine what it will do with 27ft/lbs of energy kicking back from your recoil. Take a hammer to it... put that thing through hell. Try to find it's breaking point. Only then can you know what its limitations are and whether it will withstand a magnum cartridge. Have you handled the HS Precision stocks? They are literally hard as a rock but not terribly heavy by any means. That is what you're looking for.

So you're going to take the foam core and carve it into the approximate shape, then plaster of paris around it to create a mold... then fill said mold with the modeling foam to create the stock... correct?