Dr. Scott M. Baker made a Masterplay clone:
The Masterplay was originally a device that allowed Atari 2600 digital joysticks to be used with the Atari 5200. In this post, I describe a masterplay clone that I created.
The analog controllers for the Atari 5200 are (in)famous for their poor feel and performance. The analog stick, while a really good idea at the time, doesn’t have a decent return-to-center feature. As the vast majority of the games for the 5200 do not make use of the analog capability, this feature while innovative, is far more of a drawback than a benefit. As such, there have grown to be a number of different solutions for connecting Atari 2600 sticks to the 5200.
See the full post on Dr. Scott M. Baker blog.