This is a pretty simple project. I hand wired a prototype and it works fine. It does require modding the Atari to bring Reset and Select to an external connector. For my prototype I just used TRS audio jacks for that. Once that's done it's connected to the adapter so that Start and Select on the controller serve as Reset and Select. I did this for a few reasons. First of all even though the Atari 2600 was my first ever system I've been playing with a D pad since 1985. After all this time it's how my brain knows how to play games. Secondly I wanted to make it convenient to get to Reset and Select without having to reach over to the console. Also the Reset and Select buttons on my 2600 jr are aging and work poorly. I chose the Neo Geo controller because they work in the same manner and really only require a pinout mod. Now I can sit back and play with a Dpad and don't have to get up and reach over to the Atari to press reset and select. I'm fine with the hand wired prototype, but here are some simple PCB's I have on OSH Park in case I want to do it again.
I made a video too. https://www.youtube.com/watch?v=o8yyBfNLjhY