Not that you necessarily need it to withstand a whack from a hammer... but I would rather know what its limitations are prior to building/firing.

May I also add... that when you eventually mate the stock with an action and go to fire... mount it on a durable sled, and tie a string to the trigger... I'd want the first few shots done away from me so that in the event the stock does break... you're not holding it against your face.

The more layers of screen/mat you can use the better... that is literally what will give it its strength. The resin just bonds fibers together.