Look at the various shotgun sights for sale at brownells or midway etc. You might find something that may work. You will probably have to settle on one of the magnetic type sights as im not sure the thin barrel on the axis would take well to being drilled and tapped but im no gunsmith so i could be wrong. Any gunsmith would be able to hook u up but it would probably cost atleast half of what u paid for the gun.
Kill em all and let God sort em out! USAF