Itachi's Necklace
Pretty simple design and print. You print 1 half 6 times to get 3 complete pieces.
Be sure to scale the pieces to match the thickness of your necklace. Then you simple run your necklace through the grooves and glue the other half on top. Make sure the spacing on the necklace is correct before gluing. I found it easiest to start in the middle of the necklace and put the other 2 equal distances from that one.