This is an oldie, but deserves some new attention.  Over at Grand Idea Studios, they have files for the Atari 2600, Atari 5200, Atari 8-bit computer, and ColecoVision consoles.  These files allow you to build their own homebrew video game cartridges.

If you decide to work on an Atari 2600 game, check out Thingiverse user joegrand’s 3D printed Atari 2600 cartridge.  The Pixels Past boards fit perfectly inside.