See if you can find one to borrow for a while to begin with. You might not like what it does to the balance of the shotgun. Most of the 'working' 870s here are without extensions.
If I were forced to use one, I prefer Remington's factory version. Choate is a reasonable second choice IMHO. And use a barrel clamp, too... not because the extension needs it, but because the threads on the magazine tube do.
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.