Williams Gun Sight makes a set of clamp on sights for shotgun ribs. Two versions, one with firesights and one with regular black rear/gold front IIRC.
I have a set of the firesights that I used for a while on an Ithaca pump when I was hunting in a shotgun only area. Nice setup for quickly converting a bird gun to shoot slugs. The Williams are adjustable for W&E. I shot probably 40 or 50 rounds (12 gauge) total and they held their zero.