Good project, just dig in! its very fulfilling when it comes together.
Heres a pic of my half cab / wall-cade, inspired by the same polycade you linked.
Its runs a raspberry 3b with AM, generic pc speakers, a hdmi 19" screen, usb led-lights and double dragonrise usb-pcb´s.
I made the plans in Illustrator and built it at work with our cnc-table.
really only got two good tips;

buy a multi outlet with a couple usb sockets. I draw the power to the lights and the speakers from there, so the raspberry dosent overcook. If youre green on linux/mame etc; Read the Retropie-wikis, watch lots of youtube! :)

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