18650 lithium cells are great - high capacity, higher voltage and current than household batteries. But you don't want to just throw a lithium battery in to your bag because they can be a fire risk if shorted.

I saw a simple 18650 holder that was two caps that were held together with elastic bands (https://thangs.com/designer/Nf1nk/3d-model/18650%20holder%20%281%29.stl-861781). However, I also have 21700 batteries, and this one wouldn't work with those. So I whipped up a simple FreeCAD project that could be easily modified for most common sizes.

Print two and insert up to three 18650 batteries. Secure the two caps with hair elastics.
The original had no license specified. It's a pretty trivial object so do with it what you will. I am also uploading a version for 21700 batteries.

Prusa

I3 MK3S

No

No

0.2 mm

15%

This is just how I printed it - use whatever settings you like