The TRUGLO .410 Bore Shotgun Glo-Dot II Fiber Optic Front Sight is what you need for your shotgun. This fiber optic sight is green, and is designed for plain barrel shotguns.
Specifications and features:
.410 Bore shotgun fiber optic front sight
Designed for plain barrel shotguns
Increases visibility and accuracy in low light shooting conditions
Endorsed by Ducks Unlimited