The UnoCart is the Atari 800 version of this:
https://github.com/robinhedwards/UnoCart
Uses an STM32, a tight loop responds to changes on the Atari's address bus to emulate a ROM, all through STM32 GPIO. Atari 800 has a 1.8 MHz bus, but it's still cool that microcontrollers are fast enough for this to work.
Alternative straightforward way is to use an FPGA, the UltimateCart: