Based on the plans here, which I'm using as a starting point, I made a layout in Visio of a 48x19 control panel.
I used information from Ultimarc's website to make sure I had the sizes of all the parts correct. The Ultimarc buttons need a 1 1/8" diameter hole, while the actual surface area occupied by the button and the plastic surround is 1 5/16". Holding an imaginary joystick in one hand and resting my fingers on the kitchen table, I decided that 1 1/2" center-to-center made for good button spacing.
I've also planned for a spinner, even though I haven't ordered one. The Optipac board can interface with one, so I left space for it above the trackball. The trackball buttons will double as the spinner buttons. I like the look of the Vortex spinners from Oscar Controls, so I made plans for an appropriately-sized hole, with a view to ordering a spinner "some day".
By the time I finished the Visio work, my parts had arrived, and sure enough, no duty or taxes to pay. Nice. I hauled them all out of the box and tried out the buttons and the joysticks, but after that, they sat around for a few days.
I cut out a 48x19 piece of 5/8" plywood to use for the panel, but I wanted to prototype things before starting to cut into it.
So I made a double-layered cardboard template out of some old boxes in the basement, and pencilled in the locations for all the components.
Then, I cut out all the holes, and installed the parts. It's far from elegant-looking, but gives a really good idea of how the whole thing goes together.
I used the Neo-geo colour layout for the buttons, and tried everything out to get a good feel for the joystick. I want to top-mount the joystick, if possible, to get the extra height. I tried the joystick mounted from below (along with a 1/2" offset to account for the thickness of the plywood versus the cardboard), but it was just too low.
So everything seems pretty good. The distance between the two joysticks is a bit large to comfortably play Total Carnage, Robotron or Battlezone, but I figure the firing for those games can always be mapped to buttons. I'm not a purist, and because I've only got two joysticks anyway, two-player Total Carnage is going to require firing via the buttons.
With the prototyping done, the next step is to start cutting holes in my piece of plywood...