I made a Bluetooth boombox of some old speakers.
The speakers were free on Kleinanzeigen (I guess kraigslist or whatever it's called would be a similar thing).
The Bluetooth amplifier board (TDA7492) I still had laying around.
The 3D printed battery adapters are for my 18V Ryobi tool batteries and for my 36V Bosch lawnmower batteries.
The Bluetooth board uses 8V to 25V so I had to put a 40V to 20V DC - DC converter inside for the Bosch batteries.
Some old belts were sacrificed for carrying straps.