Combo 18650 and 21700 battery case. I made this because I needed a way to carry an adapter for a 21700 battery and two 18650 batteries. But a 21700 battery fits in just as well as the 18650 + adapter, as that's kind of what the adapter does.
Fit is very snug as I didn't put ANY play between the top and bottom cases where they fit together, so you can go with friction fit if you like or just take some sandpaper to the edges where they meet and use magnets to hold them together, or a combination of both, as I did.
I printed mine in PETG and it worked perfectly.