In medieval nut and trigger locks the nut rotated inside a socket in the stock. Reinforcing the socket is - while not strictly necessary - a good idea to reduce friction and wear on the wood. The process described below is probably the easiest way to do just that.
Making the reinforcement plate Edit
First start by determining the side of the plate with a cardboard template:
Next cut the reinforcement plate to shape:
Once the plate is of correct dimensions, mark the outlines of the slot for the trigger. Unlike shown here, the slot does not have to extend all the way from end to end:
Start making the slot by drilling a large hole to one end. Then continue the work with a hacksaw:
Finish the slot job with a file and then polish the socket plate:
Finally, drill small holes for attachment screws or nails. Drill half-way through with a larger drill, so that the head of the screw or nails can be pushed below the surface of the plate:
Forging the socket into proper curvature Edit
Forging the socket to correct curvature is relatively easy, but requires care. The method I've found best is to place the plate over open jaws of a vise and use careful strikes with a cross-pein hammer to curve the it. Once forging over the jaws is no longer possible, finish the curvature with careful strikes on top of an anvil. Provided all this is done very carefully, the results should be adequate. Unfortunately there are not pictures of this process at the moment.
Attaching the socket reinforcement Edit
Attaching the socket reinforcement to the stock is easy with small nails or screws: