If their official sizing kit 3D models are accurate(better be!), there's exact amount of space for an SR721W watch/hearing aid silver oxide coin cell. And these

If their official sizing kit 3D models are accurate(better be!), there's exact amount of space for an SR721W watch/hearing aid silver oxide coin cell. And these are not even the tiniest of standardized replaceable batteries. 1: https://imgur.com/a/yupC9lN https://imgur.com/a/yupC9lN