Hello there, I only have two issues. Im all about the multiplayer nature of your pannel, but why the 3rd player with 6 buttons. I don't know of any games that really even use a 3rd with 6. I can think of some that use 4 players with 3 and 4 buttons though. My oppinion is either make it a 4player pannel and build it out from your box, or take the 3rd and make it a 4 way or get rid of it all together. If you get it out of there then you will have quite a bit more room.
My other is with the button spacing. The thing about the spacing is, just as you like the seat in your car a certain way, you will like your buttons a certain way. I have built one panel that had a 8 button layout for console/ mame use. When I built my first panel, I used the same spacing, as set in the mameroom layout program. After playing for about an hour, my hands would ultimately get sore/tired. I have large hands but I ended up getting some cardboard and some extra buttons and built a mockup, to really test the way my fingers laid. After trying a couple, I ended up setting the spacing so that the buttons nearly overlapped each other, that seemed to be the most comfortable way I played. I then measured that, re set it in mameroom prog, then exported the file to a dxf for autocad, and had that plotted out. There is no magic layout or shape, just what works for you.
As far as the design goes, I think its coming along. Its so hard to come up with a good design for a project. Good luck and good work!