Could you use a piece of thick leather to make the "D loop" backing and attach the Tandy Celtic conchos to the leather backing? Or you could take a metal "D Ring" loop leather around the straight shaft of the ring and attach the Celtic conchos to this. Also based on your design you could just use a leather FOB.

Last try flea markets and antiques shops. I have seen old watch FOBs that might be interesting for this purpose.