For this adapter, you will need to be able to machine the front and rear spacers from aluminum or brass. The dimensions are given in the drawing.

The two (2) C cell alkaline batteries are replaced by one (1) 18650, button top, lithium battery. I have used the unprotected type. The 18650 provides 3.7 volts compared to 3 volts from two C cell alkaline batteries. I am currently using a 4.5 volt 3 cell Mag-Lite bulb which works quite well, but I intend to replace it with a LED. LED bulbs are available on eBay and are expected to be the bulb of choice.

Tips are gladly accepted and much appreciated.

The following video explains the design, demonstrates assembly and installation into the Mag-Lite. https://youtu.be/6BcD2K7EjpE

The 3D printed adapter holds the battery and spacers in the correct position and acts as an insulating sleeve. The adapter, with front and rear spacers, provides positive contact with the Mag-Lite switch contact and the tail cap spring.

The C cell alkaline batteries are rated 8000mAh. Alkaline batteries are expensive, tend to be undependable and are known to leak, which can ruin your flashlight.

The single 18650 battery is rated 3000mAh. It will last longer than alkaline C cells when running a LED bulb and I expect the 18650 will hold its charge (unused, sitting on the shelf) 6 months or longer.

I have designed adapters for 2C, 2D, 3C and 3D Mag-Lite Flashlights. The 3C and 3D adapters hold two (2) 18650 batteries. To avoid confusion, each adapter is listed on Thingiverse separately.

Other Mag-Lite adapters available:

Mag-Lite 2-Cell D Size Adapter for 18650 Lithium Batteries
https://www.thingiverse.com/thing:3473302/edit

Mag-Lite 3-Cell C Size Adapter for 18650 Lithium Batteries
https://www.thingiverse.com/thing:3473408/edit

Mag-Lite 3-Cell D Size Adapter for 18650 Lithium Batteries
https://www.thingiverse.com/thing:3473442/edit