Pi + Mumble + iCW

I commute to and from London 4 days a week.  There's an hour on the train each way.  I can only do so much CW training. What I need is something that connects to the phone to allow an easy CW QSO or two.  iCW is close, but doing that with a computer and all the gubbins is a bit too much to set up.  So I was thinking about this:

What sort of small black box can I make that will wifi to my phone hotspot, have a plug for key and headphones and micro USB for power (or have battery brick inside.)  It'd have just one big knob for channel selection and a couple of LEDs to indicate connection status.  

What would that be?  First thought is a Raspberry Pi Zero W with the headless Mumble client by Daniel Chote: https://github.com/dchote/talkiepi and a bit of work to interface a key and add a hacky headphone output.

So, that's the basic idea.  I'll see where experiments take me.