Can't get more simpler than a printed tube, screw and a nut. I used this as a pass through for my Harbor Freight small flashlight to use a 3.7V lithium rechargeable cell. Originally two 1.5V cells could provide as much as 3.2V but a fully charged lithium can be 4.2V. If the flashlight does not have constant current circuitry, I will be overdriving the LED. Printing two of them with one 3.7V battery will work fine in a three-cell device.  Be cautious on your application.

I used a 45mm x 3mm button head screw and sanded off the threaded end to the point that the nut set flush with the end of the thread when assembled. The PK Cell is rated for 350ma. I ordered ten from eBay. I tested all ten and got a range of 368ma to 399ma.