I bought 2 cheap "micro USB 18650 TP4056 charger" from ebay. Connect their outputs in serial and use two separate telephone chargers to feed them with 5V. You get then : (BAT+) (BAT- connected to BAT +) (BAT-)
insert the two chargers in the case and glue the cover. That is !
I use it with my DIY mavic mini batteries. It does a perfect job and balances the cells https://www.thingiverse.com/thing:4544954