Get yourself a brass, corner brace from the local hardware store.I hope I dont ruffle any feathers by asking this but are any smoothbores specifically flintlocks produced with a rear sight ? I recently got a Pedersoli Brown Bess that was in somewhat poor condition in a lot of muzzleloaders that I bought from a deceased neighbors wife. I cant seem to hit the broadside of a barn.
You cut it down and attach it using the tang screw. You file a groove in the upright portion to form your rear sight notch. This doesn't harm the musket, but should help you to get closer to the target.
I will send you a message about your Bess, as well,
LD