You could only use it on the same battery, of that specific phone. So you design a thing, 3D print it, OK and then what? All in the name of environment friendly? Yet you did have to design it, get the PLA, solder it, get it shipped from China, etc.
My wife has a universal battery charger, and this charger supports all kind of standard rechargeable batteries (AA, AAA, etc etc). But it does not support smartphone batteries. Or any other non-standard, proprietary format. To access those, they have a standard: USB-A, microUSB, and USB-C.
Also, if you want to save your battery life time, you’re better off disabling fast charging unless when you need it. Of course, it is enabled by default, cause of planned obsolescence.
This video also basically shows how Scott builds his own powerbanks. Just not with a beautiful case.