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. But for these I thought that may be a bit much given their heft. 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.