Pilot the hammers with something like a 4.5mm drill bit then swage it out to the proper tapered square, i used some 5mm x 5mm tool steel to make the punch filing a taper into the 5mm x 5mm blank and filing it to round at the end, that way you can put the tip of the punch into the pilot hole and visually put the square edges of the punch in the correct orientation on the hammer in the down position, or full cock position, then just hammer the punch through swaging the hole to square.
This method will give a very good tight fit, make sure you castings are annealed soft and you punch is good and hard.
I have used this method to fit replacements to two pinfires and a percussion gun, with great success.
If you mess up or are unhappy you can weld the hole up and try again, i have had to weld two up and refit them, only because the orientation was slightly off on one hammer, and on the other one i had previously tried to fit by filing a square hole.
