18650 to C-Cell positive terminal adapter

thingiverse

These adapters make it easy to fit a standard 18650 cylindrical lithium-ion cell into a C-cell battery tray. To replace two C-cells in a row, use the long version of the adapter in the tray. For a three-C-cell tray, you can insert two 18650s with one normal adapter each, but be cautious! Two lithium cells produce significantly more voltage than three C-cells, and if not used correctly, they can damage electronics. A spare spacer ring for the negative terminal is highly recommended.