This is a 2D Cell to 18650 battery adapter. Designed to fit into a 2D Maglite but may work in other applications as well.

The negative side is where the washer goes and that's what contacts the spring on the Maglite flashlight. The washers need to be 7/8" or larger to make contact with the cap spring.

Will need hardware below that's available at most any hardware store. I designed it for different screw sizes you might have.

2" #10 Screws

https://www.mcmaster.com/91440A205/

2" #8 Screws

https://www.mcmaster.com/91440A205/

2" #6 Screws

https://www.mcmaster.com/91440A159/

3/16" 1" Fender Washers

https://www.mcmaster.com/91090A120/