I had a Germ. Martingale years ago, that I was given when I was in the UK,called a Market Harborough, I believe . The reins had small 'd' rings (3 on each side), and the martingale like the running martingale had small snaps on them to attach through the bit rings to the 'd' rings. I also remember a donut where the snap straps attached to the breastplate.
I think all you'd have to do, would be take off the rings from the running, add snaps, and have 'd's' on the reins.
Not sure even if I still have mine. I will look and see if I do.
You can buy the D ring reins http://www.vrtack.com/leather-reins-...ngs-p-833.html - stateline tack has some cotton web ones - then buy the round white cord at the hardware store and snaps and tie what you need. I actually prefer the cord b/c it runs better thru the bit, esp better than a cheaper German martingale set that has stiff leather. Very cheap and affordable to make your own! And it is so light weight that when you remove it you can put it in your pocket.
The truth is what you can get other people to believe.