I'd go with a small base that as a 'pinned in place' blade on a base.
The blade style,,well that's your choice of course.
Can be 1/2 moon, made from a coin of your choice.
Can be any style ramped blade, or even a patridge style.
Gold bead the face of a patridge style would be a nice match for the rear sight.
A base can be made small enough and w/a dovetail to fit into a very shallow dovetail in the bbl.
Dovetail attachments don't have to be very deep to be effective in holding things together.
The sight base is fitted tightly to the bbl dovetail, then sweat soldered into place. Then the base is shaped and polished as one piece with the bbl.
You get to add the gentle swept contours to the base coming up from the bbl surface that way and that adds a lot to the visual effect of the base and & bbl being one piece.
It won't show as two pieces if fitting is done carefully. Yes some touch-up blueing will be needed when completed, but that can be done with rust blue so the entire gun is not effected.
Artificial wear to the new sight assembly and muzzle brings everything back into blend.
Here's one made and fitted to a .45cal Ruger SA bbl. The bbl has been tapered down from factory size so it's not very thick at the muzzle.
Even so the dovetailed assembly of the new sight base can be done. There is plenty of metal to work with.
The base can be narrower than this one of course. Any overall dimensions are your choice when doing custom work,,that's the nice part of it all.
You get to decide what looks right for the gun.
This one has a 1/10" width blade. I just happen to like them that way.
The blade got a flush gold dot on the face after fitting and before the bluing.
Make the base as high or low as necessary . It only needs room for the cross pin and enough depth for the slot to hold the blade in place.
You can see everything was roughed out oversize when being made.
Sight in the gun while 'in the white' . Then remove the blade to add a gold bead if you want that feature.
Then re-attach the blade,,do a final light touchup polish and then reblue the muzzle end of the bbl w/ sight attached.