For those ordering the D-ring get the extra O-ring for it, works great. For an extra trick I rounded the edges with a Dremel tool so I don't chew up cord put through it or gouge my hand when I tighten the tail cap.
Wow.. glad I found this thread.
MagRingC kit x1= US$15 (18650 tube(100mmL) with O-ring), retain ring, copper strip, battery pad & MagRing2(Sandblast finish))
21.25 sent (+3% for CC payment)