I have a few of these UV flashlights, https://amzn.to/3KuQ927 They work great with AAA batteries but I wanted to use my 18650. It's a little brighter and easier to recharge. While it works fine just dropping the 18650 battery in there, it works better with a shim. These take about 12mins to print with 15% infill at .28 layer height. Print outside - in for dimensional accuracy. There is a slight chamfer to help the battery into the hole. Slide it about half way down the battery and it won't rattle in the flashlight.