Today I have some relatively new information on a $17 controller adapter that you can obtain on Ebay that, when combined with a PC 15 pin gameport to Atari 5200 controller adapter (like the one currently being made by bohoki) will get your Atari 2600, Sega Master System, and Sega Genesis controllers working on your 5200. What's extra awesome about this is that the Sega controllers have compatibility with the two button games, and this is what the Masterplay Clone enables. (Unfortunately, no one is currently producing the Masterplay Clone for purchasing, though.)
Here is the Ebay link to the product: https://www.ebay.com...f4AAOSwUd9aaMnL
I've ordered adapters from this Ebay seller, Ikonsgr74, since 2015 and can vouch for his top-notch workmanship. He will not disappoint you.
I have some additional good news: The Atari 2600 Trakball works with it to play the digital games that may be worth a try on the 5200. I have tried it with Gyruss, Ballblazer, and Galaxian with nice control results- especially Gyruss. The trakball unfortunately doesn't work with the analog-only games like Missile Command, Tempest, Star Wars Arcade, and Super Breakout.
For two button digital games like Defender and Dreadnaught Factor, the Sega Sportspad (which is a trackball) also works great.
If you have a couple of Camerica NES products (The receiver for the "Freedom Stick" with Genesis cables and the "Freedom Connection" transmitter device) you can add those adapters and get your NES controllers working on the 5200. I have enjoyed playing with my NES Starmaster joystick and NES Max gamepad on games like Dig Dug and Defender. In fact, on Defender, the Star Master joystick helped me to get my best ever score of around 89,000. That joystick has a very satisfying 'click' for each direction it is tilted- though it's not actually a microswitch-laden joystick like the Quickshot II Plus. Hard to describe, but it's just a perfect joystick and I'm very grateful to be able to use it to play my favorite 5200 games. I've detected no lag by the way, so that's good...
Also there's a way to connect the 8bitdo NES Retro Receiver to the 5200 via the two adapters (and Camerica NES products and NES Fourscore) and play your 5200 digital games wirelessly with the controllers it's compatible with: The Dual Shock 4, Dual Shock 3, Wii Remote, and Wii U Pro Controller. I've tested it and had great success just last evening.
Hope you enjoy the info. Special thanks to Swami for being the 1st guinea pig to try out Ikonsgr74's newly revised controller adapter on the 5200 (via being added to the bohoki adapter). I'm definitely grateful for learning about this from you.