I'm glad I spotted this message as I have been looking for the same thing for my 686. If you find anything please post again here and I'll do the same.
I know Ron Power use to make one as he did a lot of PPC guns and equip but he is retired now and will not do anymore. One of my fellow shooters knows him. I heard his son has somewhat taken over and has some Ruger/SW stuff on a site http://www.powercustom.com/
If you come across anything elsewhere please let me know as I'll do the same.