I had a temporary rear sight approach described in this thread:
https://www.muzzleloadingforum.com/threads/temporary-rear-sight-for-smoothbore.101944/
My latest approach is to take some small "rare earth" mini-magnets (there are different brands available at hardware stores, Home Depot, etc.), which are in the form of small circular disks with holes in the middle (some models have holes and others are solid across). I believe that the ones with holes are referred to as "rings" on the box. They are extremely strong magnets for their size. Different models of the disks have different outside (and inside) diameters, and different widths. Here is a link to one Home Depot model; it's not the one I have ended up using but it illustrates the concept:
https://www.homedepot.com/p/MASTER-...th-Magnet-Discs-12-per-Pack-07090HD/202526371 One day, I was using one of these magnets for something else, and quickly realized that it would make a great peep sight (being similar in size to one on a M-1 Garand).
After experimentation at the range and trying lots of different combinations (which I won't bore you with now), I realized that the best approach (at least for me) was as follows: (a) use the model with the smallest outside diameter but with a decent-sized middle hole; and (b) take all or almost all of the entire set that came in a package (perhaps 8-12 depending upon the make and model), stick them all together in a straight line, and then lay them down on top of the barrel near the lock, centered. When looking through the hole, you are actually looking through many small magnets, but, if properly lined-up (which you can readily see from above), it seems like a regular peep sight and thus goes out of focus as you focus on the front sight. The magnets are very strong , and won't readily move when handling the gun (particularly in a line of 8-12 together). After shooting, they sometimes (but not always) get slightly out of line, but it takes a few seconds to fix that (which I typically do after reloading). This magnetic peep sight is more for the range than for hunting, since you can more readily double-check they are lined up properly at the former than the latter.
As with any other peep sight, accuracy depends (among other things) upon the height of the peep sight, the size of the peephole, and the size of front sight. After experimentation, and (frankly) some luck, I was able to get the peep sight to be dead-on at 50 meters with my favorite smoothbore flintlock (or at least as dead-on as I can be - ha), without doing any filing of, modification to, or replacement of the front sight. Of course, you can adjust windage by just moving the magnet-line slightly to the left or right.
This is a temporary fix. I put the magnets on when I want to shoot a smoothbore, and then take them off when finished with my range session (putting them in a container in a range box). I've noticed no scratches or other damage to the barrel from the magnets in my experience with my guns (but your mileage may vary).
The magnets aren't as stable as the adhesive button mentioned in the above-referenced thread, but allow me to use a temporary peep sight on a smoothbore. As I get older, peep sights help my "old eyes". It might also be an useful approach for others.
