I just bought a Choate mag extension kit for my 870. The kit came complete with the extension, a high-vis follower, new spring and the clamp that holds the tube rigid in line with the barrel. The clamp has a sling swivel integral with it.
The parkerized finish was a perfect match to the gun, but you can also get it in blue.
They make the kits for:
18" - 7 shot
20" - 8 shot
24" - 9 shot and
26" - 10 shot.
Made for many models, including the 1100.
Price for the 7 or 8 is the same at $37.75 (plus S&H) direct from Choate at
the 9 costs $2.75 more.
My only complaint with them is they promised same day service, but took 10 days to get mine out the door. Once I got it though I forgave them when I saw how good the quality was.
Hope this helps.