I hacked this together in record time last night.
Ingredients:
4W solar panel: http://www.solartechnology.co.uk/solar-panels/fold-up-solar-panels/fold-up-solar-panel-4w
T-class amplifier: https://www.amazon.co.uk/d/9nm/LEPY-LP-2020A-Amplifier/B00C2P61FO
6Ah lead-acid battery: http://www.halfords.com/motoring/motorcycling/motorcycle-batteries/halfords-htx7l-bs-scooter-battery
12V to USB converter: http://www.halfords.com/technology/sat-nav/sat-nav-accessories/halfords-multi-charger-high-speed
Random wires, buttons, switches etc. to taste
No photos of the build in progress cos I was too busy. Stayed until 1am in the end which is way past my bedtime on a school night these days! So here’s the result:
This is the stable position. The battery is at the “bottom” in this orientation.
I wanted to add some legs to prop it up at an angle but pfft.
USB sockets by the green light, activated by the switch. Hold down the button to get a voltage reading on the battery.
Amplifier controls and connections (I de-soldered the connectors from the amp and re-wired them round to the front). Speakers sold separately, or mount the whole thing inside a wheely suitcase with speaker divers mounted in it:
(suitcase itself currently in for repairs)