For tensioning chain, we had great luck with these this past year:
http://www.zoro.com/i/G0347697/?category=6001
Slipped off twice that I can remember and it was about a minute to get it back on and we added a little extra tension (they are adjustable) after to take care of it.
We were happy with them. They are also completely off the shelf so we'll be able to use them again.