Cases that hold 4 batteries each, for AA, AAA, and 18650 sizes. They're a snug fit, use minimal plastic, have lids that snap on satisfyingly, and feature labels and charged/uncharged indicators so you can tell which battery you want to grab.

Fusion 360 (.f3d) and STEP files are uploaded too, and the model is fully parametric so you can easily make it fit any size battery that you'd like.

09/03/2024: I've updated 18650_case.stl with a new version with slightly increased battery slot sizes following feedback about a tight fit from a few users. This newer version should fit 18650 cells with double wrapping and with built-in protection circuits.