How old
is your 686? Most newer ones, and I can't remember how long back but I think 94 (?) come tapped from the factory for scope mounts. Open cylinder and turn upside down and you should be able to see the screw holes.
Check Brownells they should carry something from some of the custom smiths who are now parts suppliers. Most will have a weaver rail that will already fit the pretapped holes. I'm at work right now otherwise I'd look up something for you, when I get off I will. But you'll be able to find something.