!! WARNING: I have no idea what I'm doing !!
I've decided to make this public in the rare event it might help someone.
This is a very simple, likely very wrong (EP)ROM emulator using a Teensy 4.1 and Arduino/PlatformIO. It works for my purposes (emulating an EPROM for use in an Atari Lynx EPROM cartridge), but it it is probably not the "correct" way to design hardware. I have also used it on a Game Gear EPROM cartridge, but it was not heavily tested.
Please take caution with using anything you see here. It's my first time using KiCad, my first time designing a PCB, and my first time creating hardware. Again, it works for my purposes, but perhaps it'll burn your house down.
Enjoy!