Here is the first iteration of my case:

Tomorrow I will get this cut out at work and stick together; I have added a cut out for the Pi ports on the back, but will need to drill out the sockets for the xlr/Jack inputs x2 and the jack sockets  out x2 and a headphone jack out.

The analog expander chips I have ordered won't work with the Pi (I need an analog pin to expand of course) so a new chop will need to be ordered.

Not to worry though, as my expander chips will be getting put to use with an Arduino Pro Micro to make a simple electronic drum brain.