Most shotguns come from the manufacturer with an appropriate sight configuration affixed. In cases when sights fall off or the shooter determines another type of sight is preferred, replacement or construction are options. Constructing homemade sights is generally discouraged because of safety and liability concerns, however some work is possible.
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To manufacture a bead you can machine a post down to a beaded top, or simply insert an appropriately sized post into the barrel. Either variation can be soldered or screw-fitted into place. You also can mount a second bead slightly behind the front and use the "double bead" sighting method.
Blades are substantially more difficult to fabricate. Tap the barrel for two screws, as the length of a blade requires more stability than a post or bead. Blades can be constructed in several designs, including fibre optic. Cut a small fibre optic tube and affix it to the blade base. The cross section of the tube brightens up and makes it more visible.
If you want to modify your shotgun to shoot slugs or buckshot more effectively, you can mount a rear, rifle-type sight. A rear sight should be affixed with two mounting points so it does not pivot.