I used these guides to decide my layout, started simple but managed to squeeze a mouse based trackball in the middle after moving things around. Still doesn't look cluttered
Standard six button layout, plus two extra per player for 'shoulder' buttons ZL/ZR so I don't also have joypads laying around so ended up with 8 buttons per player that wasn't my original plan
https://www.slagcoin.com/joystick/layout.html
https://retropie.org.uk/forum/topic/5220/arcade-buttons-how-many-layout
This was my original layout, but as I also wanted true pinball emulation (the PI simply can't handle anything like Future Pinball or Visual Pinball) I also added two buttons on the cabinet sides for pinball flipper and also load/save states for emulators that support it
Hence finally going down the IPac4 route. Worth the extra effort as its a true keyboard interface that supports multiple buttons and upto 4 players or in my bartop with lots of buttons and two players. BIG improvement over the Chinese USB interface that the kit I bought came with
https://www.arcadeworlduk.com/products/I-PAC-4-Keyboard-Encoder.html
This is basically the joystick/button set I used, but had to rewire the joysticks to fit the I-Pac interface. Not difficult
https://www.ebay.co.uk/itm/USB-LED-Arcade-Mame-DIY-Kit-2-Joystick-20-LED-Illuminated-Button-Encoder-Set/143568782687?hash=item216d5dc55f:g:dM8AAOSwKtVegyuu
Speakers were taken from a dead 2.1 PC speaker set and bought a cheap 2.1 amplifier that runs off 12v from China. Its rubbish and needs changing for a better quality 2.1 amplifier but it was a start
I'll post a picture of my bartop later today sometime. The vinyls and edging need adding but this is the case I used for my PI bartop, I just squeezed a PC motherboard and power supply inside when I decided I wanted pinball emulation and more intensive emulation the PI will never be able to manage such as PS2/Wii/Wii U/Xbox
This is the original layout I used, but added a trackball mouse. The PI can easily support USB hubs but make sure its a powered one and one that doesn't reverse power. A lot of cheaper ones do so send power back to the PI, Buying one from the PI HUT will guarantee it works as it should
!(https://retropie.org.uk/forum/uploads/files/1484412665733-img_2320-resized.jpg)
This is the trackball I used, but for the PI you will need a PS2 to USB adaptor. It works exactly as it should so I can also play trackball games as they should be played such as Centipede and also it functions perfectly well as a mouse, but I had to add yet two more buttons to my cabinet panel to use it as a mouse when needed. Amiga games need a mouse and this works perfectly for them such as Cannon Fodder. Amiga emulation will be no problems with a PI4 as it worked on a Pi3B easily
https://www.ebay.co.uk/itm/Mini-Illuminated-50mm-PS-2-Trackball/184018227197?hash=item2ad8574ffd:g:Ky4AAOSwqCtdwdFv
Also found a few things you might find interesting. The passive aluminium heatsink case for the PI4 looks interesting. I would consider adding a standard PC case fan on top as the guide says the casing WILL get hot and make it run even cooler with an extra fan. Should easily power the fan off the PI itself
https://thepihut.com/collections/all/products/aluminium-armour-heatsink-case-for-raspberry-pi-4
https://thepihut.com/products/usb-2-0-powered-hub-7-ports-with-5v-2a-power-supply
https://thepihut.com/products/4-port-usb-hub-usb-2-0