This adapter replaces two D-Cells with one 18650 battery. I use it in my Mag Flashlights. You will need two battery tabs I use Keystone 209 clips from Amazon.
...Plus The negative end I am using a quarter, but you can use a washer as well. I just didn't found it easier to solder a quarter.
http://www.amazon.com/Keystone-Electronics-Contact-battery-spring/dp/B00LPOVJHS/ref=sr_1_2?ie=UTF8&qid=1451248043&sr=8-2&keywords=Keystone+209
