I salvaged an amplifier board and speakers from an old monitor some time ago.  Having looked it over, it's very simple to use.  One jack is stereo in, the other for stereo out (headphones?).  Speakers plug in via a socket and there are connectors for power and volume control.  Volume is controlled by a voltage beween 0 and 5v.

The only awkward thing about the board is that it needs to be powered by somewhere between 10 and 18 volts.  I'm using a USB power supply so I only have 5v available.  I will be adding cheap step-up converter to power the board, but in the mean time I have tested it as working with a separate 12v supply.

Fitting the case

A VHS video case is about the right size to fit all the components.  The idea is that in use, the lid will be lifted like the lid on a grand piano.

It might be a little snug, but everything seems to fit. I still need to add the postage stamp sized DC-DC converter and all the wiring. I need to make holes in the case for the following things:

• Volume control
• Line out jack, left
• Line out jack, right
• Headphone mini-jack, stereo
• Line out mini-jack, stereo
• USB power
• USB OTG
• Power switch(es)

I may need to work out how to switch between charging the power bank and powering the device directly from an external supply since the bank doesn't seem to charge whilst powering things.  If it runs for a few hours after each charge, then I'll leave it to run on battery only.