I think the best bet is to get one that has fast charge and buy a second cable to leave it attached. Or something like this.
Galaxy S8 Plus Battery Case, 5200mAh Rechargeable External Battery Portable Charger Protective Charging Case Juice Pack Power Bank Cover for Samsung Galaxy S8 Plus (2017) (Black) https://www.amazon.com/dp/B06Y63HPC...ag=hawk-future-20&ascsubtag=UUacUvbUpU5777338
https://www.amazon.com/dp/B01F8IRIN...ag=hawk-future-20&ascsubtag=UUacUvbUpU5796437
^ The one I'm considering.