2 Packs of Aplus Universal Zipper Zip Travel Carrying Hard Shell Waterproof EVA Case Storage Bag for 4 x 18650 Batteries (Also Fits 20700 and 21700 Battery, Charger, Cable, Earphone, Flash Drive)

21700 will fit, but barely. It's a bit of a fight to get them back out of the case.

Right size

Case fits two 18650 or 21700 batteries. The 21700 batt is very snug.Two stars because the zipper already broke on one of the cases... I opened and closed the zipper maybe eight times.You might want to try a different case as these are not built to last.