I made this boombox in sizes which fit in standart bicycle's bottle holder. I have added spectrum on MSEQ7 which made years ago.
Boombox's parts list:

1. MP3 module
   https://ru.aliexpress.com/item/5V-Slim-Mini-Size-DIY-MP3-KIT-WAV-MP3-Player-Audio-Decoding-Decoder-Module-Board-With/32594145649.html?spm=a2g0s.9042311.0.0.iR0kOQ
2. Bluetooth module BK8000L
   http://s.click.aliexpress.com/e/iznH8JI
3. 3W amplifier
   http://s.click.aliexpress.com/e/bMDpHKQ8
4. 3W 4 Ohm 40 mm speakers
   http://s.click.aliexpress.com/e/zzaDZWo
5. Step UP module
   http://s.click.aliexpress.com/e/bRHUw2ZK
6. 3-Way switch (got it from broken casette player)
7. 18650 Li-Ion element + holder
8. Spectrum indicator (i made it as improvisation but you can buy it, i think this fit well http://s.click.aliexpress.com/e/bIe1TRuM )
9. Charger plate
   http://s.click.aliexpress.com/e/b0sQQrCU
10. Some M3 screws and glue

That tiny speakers not loud, but it still enough and it goes to hot in max volume. Some more power I got after increase voltage to amplifier by Step up module to 5.5 V. Mp3 module and bluetooth connected through switch to battery voltage.
On 2200 mah player works about 3-4 hours.

https://www.youtube.com/watch?v=IqSWOvsY2rI