Building the cartridge flasher
For the flasher you just need the card edge connector and a set of stacking headers for an Arduino MEGA shield.
The PCB is "MegaProgrammer_2021-01-06.zip" in the "Production" folder of the GameTank repo. https://github.com/clydeshaffer/gametank/tree/main/Production
Once you have the parts basically everything fits where it looks like it does. The cartridge flasher design is essentially just a connector between the cartridge pins and the digital IO pins of an Arduino Mega.