I made my first version of these holders a few years ago for my camera batteries, and I really liked that I could see what is charged at a glance. Just keep charged ones with the contacts down, and drained ones flipped the other way.

I like to size mine so that the batteries have a snug sliding fit. For my camera batteries I usually go with just tight enough to stay put when upside down. You can use the single to iterate and get the fit where ya want it by scaling it in the slicer.

I printed mine in PETG, with a 0.8 nozzle at 0.64 layers.

If you want some additional info, and/or to check out the other variations, you can find it here.

Or if you want to brave my ugly CAD, the files are available on OnShape here.