I am glad I found this. I am currently in process of designing a 4P Showcase as well! I am still getting in parts, so you are a good ways ahead of me.
Your design is very similar to what I am planning, except I am excluding the spinner and including a trackball. I am doing U360s only for player 1 and 2, 3 and 4 will get switchable 4/8 way joysticks. I am planning 5 admin buttons, In a t (+) shape - Red on the left for exit game, Green on the right for Enter game, center three are all black, Up for Volume up, Down for Volume down, middle is dedicated shift, to access tab menu, save states, favorites list etc. This way I figure that I don't have to explain to people how the thing works, they can figure it out on their own. Dedicated start and coin buttons for each player as well.
Initially, I was thinking of recessed container 'holes' on the face of the CP, between P1 & P3 and P2 & P4, similar to how many games hold their guns on the panel, then I thought it would make what is already going to be ~4' CP even wider - so I am stuck working on a new plan for that. I am also doing some USB passthroughs in the pedestal for the CP, or on the backside of the CP to add and remove the guns (and keyboard if it were ever needed).
How are you planning to mount/store your guns?
Why did you chose MDF over Ply? Isn't it going to be outrageously heavy in showcase form-factor?
I am still deciding on pinball buttons - How does pinball look on a horizontally mounted TV (I am planning on a 42-46" TV)?
- I am thinking that I want to find a way to include it, but I was assuming that including pinball means I need rotation, which then brings challenges in bezel due to the drastic difference in width vs height and brings marquee challenges as well.
I am also torn on a marquee in general - I just don't think they are all that important to a MAME/Emulator Machine - since it isn't just one game, or even one system. So I am interested to see how yours looks done without it (assuming it stays sans marquee). In general - on normal arcade cabs, I don't think a marque looks quite as 'in place' on a showcase as it does on a traditional upright cabinet - but that may just be me.
Looks like a great start, I will be watching your progress for sure!
Best of luck the rest of the way.
Bill