Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4798632.
I could not find any other adapters on Thingiverse that allowed the replacement of three C cell batteries with a single 18650, many of them replace the three C cells with two 18650 cells, I believe that this is most likely to damage an LED torch/flashlight...
This is simple battery adapter to allow three C cells to be replaced with a single 18650 lithium cell.
After printing, solder a small washer to a suitable piece of wire, then feed it through hole inside the adapter (the end where the 18650 fits), the wire should hold itself in place, but if not push an 18650 cell inside to hold it in position.
Then simply cut the wire where it exits on the outside, leaving enough wire to solder on another washer to create the positive terminal.
Make sure the solder is relatively flat after soldering or adapter may not fit correctly.