Communication among briefcase halves
Use the rainbow coloured ribbon cable in a short arc between the halves.  A clear plastic sleeve will help to ruggedise it, and an IDC connector on each end lets you replace it in minutes if it wears out (make up two cables!) The design constraints you face on a project like this are different from commercial manufacturers - you can take advantage of this to make maintenance actually easier around vulnerable areas like cables and connectors. 

Plus, that ribbon cable looks awesome.

I built one of these in mahogany and brass. It was Z80 based and, unfortunately, was getting obsolete faster than I could finish it!