The main feature of this project is the nickel strip looped through the 3d print. This allows the nickel to contact the terminals of the stereo just as a set of 6 "D" cell batteries would.
I have used a BMS inside the pack to make sure the cell do not over charge or over discharge
A barrel jack on the back side allows for charger.
A suitable charger would be a 9v 2amp power brick.