I know many prefer open rear sight to be most historically correct. I know there have been threads though from shooters who (like myself) see and shoot a peep better. I know there are threads showing how others have tackled the peep sight install on their muzzleloaders.
Then today I had the idea for a peep breech. A breech plug with an upward extension that would stick up above the top flat, drilled and tapped to use screw in apertures (like those used on Williams gun sights) which are available in various opening sizes. By design, it should be pretty much dead center of bore at breech. Sight adjustment made by drifting and/or filing of front sight.
The existing breech plug on your favorite gun could be replaced with this style and if you ever wanted to return it to original just put the original back in. No other holes have been created in barrel, tang or stock.
If anyone with machining skills wants to make some for sale, let me know, I would buy one.