I designed this to keep my batteries safe from shorting out after charging.
This is for the flat top 18650 battery but the button top ones will fit if you don't screw the lid down all the way.
The pictures don't show it, but I have added a 1mm filet to the top and bottom. This makes it more comfortable in the hand and also easier to remove from the build plate.
As requested by goodwid in the comments. I've added a cap with a key ring loop.
Also added a cap with knurling nubs and separate files for the main body and cap.