Couple of small steps can make a significant different in fit for silicon shooting plugs. 1)you should have a deep impression. Tell the lab it will be used for shooting and you will have no cracks in the seal as the seal is deeper than the jaw movement. 2) use only high strength silicone in final product. Do not use instamold type silicone. If they make it right there chances are it is not going to fit well over time. 3) if you still have issues try having your impression taken with your jaw open. This will take an impression that will be much tighter as it is the largest volume you canal can be. All major labs offer a fit guarantee. Usually 3 months. If it is not working or hurts have it remade for fee. My favorite manufacture is Starkey. They have built millions of molds and know how to do it right.