I received mine today and I've been experimenting with different options on a micro cab.  I'm not sure I'm up for doing a step by step tutorial on this since I'm spending most of my time exploring potential and taking a picture ever step of the way won't be productive or useful.  Maybe if I get it right and decide to do a second cab. 
At any rate, I noticed there is an X button on the thing and don't see a single game that uses it.  Early Genesis games were 3-button which I think all of these are, and new ones were 6-button.  Granted there could be a game for the 6-button controller that only used 4 buttons, but I don't think that's the case at least with the games in my unit.  I didn't actually read the instructions to see if it mentions anything about X, but I'm pretty sure it can be eliminated if you make a micro-cab and do a custom CP.
I won't take over this thread with my build, so when I have something to show I will probably put it in a new project thread.
EDIT: After a lot of searching and not much info about these online, I did run across some pages mentioning these for sale at Walgreens.  I haven't verified, but I'll make a trip to mine tomorrow to check it out and maybe grab the Sonic one.