While this might sound self congratulatory, I finished my first cab in 2012 and after banging on it for the last three years, there is literally not a thing I would change on the CP. If the machine was destroyed and I had to rebuild it, I would use exactly the same plan without an iota of change. It is pretty much the perfect generic layout for the games I like to play. How did I accomplish this? Well, you could argue that this is actually a Mark IV design, but Mark I-III were paper only. After building the physical cabinet and CP box, I printed a full size picture of the control layout I had worked hard to come up with and spent a considerable amount of time "air-playing" a spectrum of games on it. I discovered a severe fault in my first design fairly quickly after realizing that the layout which looked so nice and symmetrical on paper had some very real practical deficiencies. Hand placement on certain controls was either awkward or would cause my palm to rest on other buttons. This resulted in lot of re-thinking and a second revised copy being printed with these issues corrected. This new copy was good—up until I started doing final measurements to drill holes and realized that in my effort to fix the layout, some of the controls no longer quite fit within the box under the panel. A little more work tightening things up produced a third copy that became my template for marking out the drill locations on the panel. It would have been my final copy too, except for the need to make a few more micro-adjustments and a last minute text label change. So a fourth copy was printed to make these changes. Only when further "air-playing" failed to find any more flaws did I commit myself to drill holes.
My full design would have had flipper buttons on the sides of the CP box, but physically I could not place these without compromising the position of the top buttons, so I never made the attempt. If I can ever find some super low-profile switches, I may add these someday, but it wouldn't really be a change in design. I have also had some preliminary throughts about building a dedicated vertical screen cabinet that would have fewer buttons, ditch one of the spinners, and reduce the overall dimensions a bit; however this wouldn't be so much a design change as a fundamentally new design for a different kind of cabinet.