I created an all-in-one portable computer that can run off battery or power jack with built in controllers. It is powered by a Raspberry Pi 3A+ using the excellent BMC64 emulator. The controllers are wired to the GPIO. It is easy to work on and adapt. I am building the power specs and space inside to have the possibility to be upgraded to a DE-10 Nano, to be used primarily for C64 and Amiga and select arcade games, though after completion and using it for a couple months, I think I will keep it as is. 12+ hours of operation. You can turn it on and it's ready in 3 seconds and it can be powered off with no shutdown cycle. The latency is very low. I'm able to play my old favorites with the same skill as when I was younger.
Anker PowerCore 26800mha Battery
Raspberry Pi 3A+
68 Key Wired Mechanical Keyboard with Gateron brown switches SK-68