Best approach?
"Don't!"
I feel for you, that's got to be a tough one - I actually think it's easier to present to a prospective 2nd, because from the world's perspective, you look (a little) more honorable: hey, at least I don't want to divorce my wife for you.
I'm sure a large percentage of Christian single women are not going to be open, regardless of how it's presented - but you knew that already. So we talking about playing the odds here - maybe taking out a full-page newspaper ad wouldn't be a bad idea.
OK, more seriously, here's one idea: make me (or some other polygamist) the "bad guy" initially. Mention in passing you have this "good buddy" who happens to have 2 wives, and that you've met them, and they're not Mormons, they fear God, and are (more or less!) normal people. If that doesn't get a wholly negative response, then you start down the "slippery slope": 1) My friend got me thinking.... 2) I want to take you on a trip and, oh, my friend and his family live nearby... 3) Here's something I'd like you read... 4) You know Amy, that cute friend of yours?... :roll:
I do suspect if you've met someone that you really care about, the best approach will be very slow, not overwhelming with the idea all at once. Just drop it at first sign of frustration, and be very patient - if she comes to really care about you in the process and see that you are serious about your beliefs and care about her feelings, maybe, just maybe, there's a chance.
Lots of prayer, wouldn't hurt either.