I believe to do it properly you have to have it fitted. Different people are fatter in different parts of the shoulder, lengths and heights of body dimensions differ for different stock designs. There are many different easy cut methods as described by 500Nitro.

A shotgun stockist should have an fully adjustable 'trial' stock but this may not suit a centrefire. I don't know. The stocker would also have to know what he was doing. Being Oz they would be thin on the ground however I have heard of one in Western Sydney.