I designed this thing so I could use 18650 Li-ion batteries in one of my small flashlights. The flashlight is the kind that takes 3x AAA batteries in a removable holder, with a tube/cap that have ~M24x1.25 threads. This particular flashlight needed a 16mm extension, which is what the STL file contains. There are also some flat spots where the flashlight o-ring seal seats, and a hole to pass 1mm wire through for ground (required for the light to work).
I printed 2 of these with Taulman Bridge and weed trimmer line (both are nylons), without supports at 0.2mm 40mm/s on my a8. You can probably get away with other filaments, but nylon will be the toughest/longest lasting by a large margin (because of the threads). For the ground wire, I use 0.7mm welding wire (copper coated) lubed with dielectric grease. Really any wire will work, but do use dielectric grease to prevent electrolysis if you use anything but aluminum wire... flashlights are usually alum.
You will typically also need to print a spacer tube for the 18650 cell to fit in the flashlight tube. I used this one:
I uploaded my spreadsheet parameterized Freecad 0.17 file if you need different dimensions. It has every dimension broken out to the spreadsheet so you can generate an extension for almost any flashlight with almost any printer (extension length, thread major/minor radii & depths for both ends, seal seat radii & depth, outer radius, battery rad, ground wire radius...). Please note that the final mesh model is an "openscad mesh boolean" that does not update with parameter changes. It must be deleted after you change the spreadsheet data, then it can be recreated. Sometimes playing with thread dimensions can also break the "tap" and "die" cuts. So beware you may have to redo those boolean operations if you play with certain dims. Please comment if you need help working with the parameterized file.
Note that there is another similar design that was a good fit on my other M24x1 threaded flashlight here:
When I found that one I was like, "heck yeah, this means I get to play instead of make cad drawings for a few hours". I assumed the configuration was more or less standard. So I printed a few extra extensions for my other flashlights. Then I opened up the flashlight with 1.25mm pitch threads and was like, "awe man, I guess I'm not getting out of it that easy." LOL! Enjoy