My Amp Ripper arrived, which I will be using for my old deck due to higher power requirements than this project, so I spent the day setting that up.  Bit of progress on wiring the keyboard and managed to program the microcontroller.  I had a pro micro in my collection and spent far too long realizing that a dodgy USB cable was preventing me from programming it.  The pro micro should be ideal it's small and should be easy to add it to the keyboard plate without taking up too much more space. 

One trick I can share: if you want to be able to get a decent connection on a thru hole plate without soldering for testing and prototyping, you can thread a fishing line or a bit of the brim from a 3d print through the hole to get enough friction to keep it solid.