Brain and Button Matrix for Stump Controller Upgrade

I'm in the process of upgrading this midi controller from being a clunky art piece that required Arduino and popcorn serial midi converter to a much nicer instrument, a Livid Instrument. I have had a brain for awhile and to get the 19 buttons all to work I need to make a button matrix. I'm going to do a 5 (pins1-5)x4(13-16) and not have a 20th button.

Here is a video of the Stump Controller in its previous incarnation.