The best built Mag extensions I have EVER seen are the ones built by ScatterGun Technologies. They have just been bought by Wilson Combat. At the present they only offer tubes for 870's but if you ask they may decide to widen the product line.
The thing is built to last... much better than most that I have seen.
Wilson even was able to coat mine with their Armor-Tuff coating instead of parkerizing... had it done in 7 days too!
Give John a call at Wilson Combat www.wilsoncombat.com
( I have no association with WC.. just a VERY satisfied customer.)