To my knowledge no one has bothered. It's easy to buy a real cart, remove the flash chip and install an sdcard socket.
If you know how to use software to design a board, it should be a super easy project. Unless you're trying to get fancy with it and add a few new features. All you need for the standard functionality is 8 traces from the first 8 cart pins to the mSD socket with spots for two tiny capacitors - if you're mimicking the official PCB. The rest of the pins after the first eight, or at least the first handful of them, are grounded.