They don’t look as good as they sound but they do the trick.

Small Adafruit stereo amps are entertaining me at the moment. 20w amp with 15w speakers in the skulls lasted 47 hours playing at medium volume on 4AA batteries. They can be powered with USB and even OTG from your mobile phone. The battery drain using the phone was about 10% per hour with the screen and wifi off.

Using the powerbank on the longboard makes them easier to transport too!

minor soldering was required and I had to cut and join a few cables but the results vs effort ratio was perfect.