Sometimes you can't see the forest because of all the trees in the way. It's the same way with cannon sights.
You need to look at sights for any firearm and there is what you need. A rear sight and a front sight.
I have seen those vernier tang sight for buffalo guns mounted on cascabel with a post front on the muzzle. I saw one guy with a Leupold 4x scope that he had mounted on a one piece base that was mounted on to a piece of strip that had two pegs that went into two holes on top of the cannnon. Still another guy had what looked like the rear sight out 1911 Colt mounted on a block with two pegs with a fixed post front. One peg fit down the fuse hole and another into a hole in front of the fuse hole. I saw one with a single pin that sat in the fuse hole and had series holes drilled up the stem for different ranges. An other guy had a split pin in the reinforcement ring of his gun for a rear and flat pin in the front. He told me he threaded the pins in tight and used a file to knotch the rear. He then start filing the notch deeper and the side down until he got the gun to hit at 100 yards. Then he filed the sides of the front sight to center his shots. His was the best shooting gun I saw.
Here are some pictures of the real thing
This is the base for sight a 30pdr Parrot Rifle at Fort MacAlester in GA. If you look at the trunnion you will see the front sight sticking up.
This is the front sight for the 30pdr. I have seen this type sight mounted on manner of cannons in the national parks and museums.
Other Cannon sights