This is a superb panel, we have had many comments asking about this since it appeared on our site.
Just to offer some suggestions which are not intended as improvements but alternatives:
Pushbuttons: James built this panel before our black-center pushbuttons were available. We now have add-on black button centers for our Goldleaf RGB buttons.
The two Ultrastik 360 FS offer full analog so some people might want to look at ServoStiks for the main 4 joysticks, as these offer a hard 4-8 way restriction which many people prefer. They can be configured to auto-switch from the front-end.
That would add 16 extra switches to the panel. So the suggested interfaces would be 2 X I-PAC Ultimate I/O boards. These would also handle all the LEDs and trackball and spinner.
One board would be mounted on each side of the panel thus eliminating any short-wire issues.
The use of the U-HID for the spinners is not really required, as James suggests. Definitely not needed for Mame. Its best to have the two spinners as different device IDs to avoid Windows randomly swapping them, but that's easily accomplished by connecting one to each of the Ultimate I/O boards. In some other configurations this could best be done by having one use its own interface and the other via an I-PAC board.
Optional extras would be:
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