A crossbar mounted bicycle sound system.

Speaker build;
4mm MDF cabinet painted with black all weather chiminea paint
2 x 6" waterproof marine speakers
2 x 10k Log Pots for volume control
2 x 40W mono amps
iBasso DX50 for the tunes

Power (Tote bag on seat post) from;
Multistar High Capacity 4S 16000mAh Lipo Pack
Configuration: 4S2P / 14.8V / 4Cell
Pack Weight: 1290g
Pack Size: 173 x 74 x 45mm

A custom built speaker cabinet hung from crossbar with double sided velcro.
Survived multiple 'unintentional dismounts' without issue.
It draws 1.5A at full volume without distortion, so this thing is loud, sounds fantastic and the batteries last for a full day of cycling about in Manchester.

My twins ride on the trailer in the back, so this set-up means I can hear the traffic better than I could with headphones and they get to sing along, too.

It also provides for a lot of engagement from the public who seem to enjoy an impromptu boogie on the pavement.