@EMDBThank's again for your advice!
I have tried to rebuild your CP layout in SketchUp based on your artwork and have modified mine a bit.
My CP layout was based on
http://www.slagcoin.com/joystick/layout/hybrid36_m.pngbut without the pinky finger buttons as I had read they aren's used so much and less is better regarding button count most of the time.
Based on your artwork I have now worked out three possible ways of adding a seventh button to allow a more natural play style for Neo Geo games.
@severdhedThank you so much for providing the images with the color comparisons!

That's what I was looking for. I had searched the forum for something like this, but couldn't find anything like it.
Seems that if I go with HAPP buttons, I'll have to stick with black or maybe yellow for the ball tops as the other colors just don't really match with the buttons. I'm very picky regarding that.
Shame that HAPP (or any other manufacturer of concave buttons?) doesn't offer ball tops.
As to the choice of the joysticks in general, I'm still not sure which option would work best for me. I do not really remember the feel of the "real" arcade buttons of the earlier days, as arcades weren't really common where I lived. I am building my arcade memories on holidays in other countries mostly... :-)
So maybe the U360 is the best option for me. JLWs are hard to find, I am not really sure where I could order them with reasonable shipping costs to Europe.
@ everybodyWhat do you think of my current CP layouts? Most probably I'll place the admin buttons on the front side of the cabinet. That would give me the option to increase the space between control buttons and the bottom of the CP to leave more space to rest my palms on. Would this be necessary or helpful in you opinion?
Gonna choose the layout and type of buttons and joysticks soon. I see there's not much to it tinkering with that kind of questions without actually building stuff. I feel it's time to start and get things done.