First: I normally HATE 90's cabinets, but I LOVE yours

Second, the new style mount appropriate for the Betson/Imperial trackball is WAAAAAAY better than that plate. Firstly, it has welded studs that are flush on top (no protruding bolt heads! WINNING!). Second, it has a nice lower 'carrier' that ever so gently cradles the trackball housing, and has a nice grommet opening for routing your wires, so they are not hanging off of the plastic trackball housing.
LINK FIXED!http://www.happcontrols.com/trackballs/55032100.htm
Its only $12, which is the opposite of what Happ usually charges for things. Usually they want ridiculously too much money; Here, it seems like they are asking too little? I would think some of the vendors on this site would have them too...
Im making the assumption that you are going to cover the panel top in plexi/graphics? If you are trying to under-mount everything and not have plexi on top , well I just wasted a lot of hot air. The mount disappears under plexi/graphics. I just routed a slight recess in my panel, so that it was about a millimeter sunken, the plexiglass bridged right over it.
I would only have 1 critique: Put in a coin door, even if it doesn't function! A cabinet without the coin door is akin to a face without a nose, something just looks 'missing' otherwise.

But i'm splitting hairs, great work so far! I enjoyed you color commentary as much as the pics, I look forward to seeing this project progress.