The best accessory is a good class at the hands of someone who knows their way around a fighting shotgun. Very few people will learn as much as fast any other way - a lot of folks complain about the supposed cost of a class, but good training will actually SAVE time and money in the long run. When you can get through a couple of varied iterations of the Rolling Thunder drill without flapping, you'll know what I mean The mindset and skillset of the shooter are far, far more important that anything that can be bought and bolted onto the shotgun.
That said, I tend to stick with Louis Awerbuck's short list of accessories for a defensive shotgun. Going back to my notes from the Shotgun I class (May 2006) -
- a stock short enough for the shooter to to use comfortably, most shotgun stocks are too long for many shooters. A good recoil pad is a big help.
- a sling is to a shogun as a holster is to a pistol. KISS as far as slings are concerned, the simpler and stronger the better.
- a white light source mounted on the gun, especially on pump guns.
- sights, if the shooter prefers them. Sights need to be simple and sturdy, front sights should be silver soldered onto the gun
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.