My two cents
I don't know if my problem was the same as yours but here is my story...
I bought Ferracci carbon cans for my 900SS and had them put on. I got them from a buddy. Once they were on I had people telling me that my gear shift side can was wiggling a fair amount. I checked it and sure enough, just touch the rear of the can ever so slightly and the whole thing wiggled around like it wasn't tight at all. Thing is, it was, at least where the strap that holds the can up was. The hanger was snugged completely yet it wiggled like crazy. The right side was fine, tight, it was just that one side. When you look closer the problem was centered on where the midpipe slipped into the header. It just didn't have a good fit there. As the pipe wiggled you could see that there was extra room down there. It was a constant annoyance to me because I take pride in my bike and have done a lot to it and every where I went someone was like, "You might want to check that pipe, I think it's loose and is going to fall off."
So, today I was bound and determined to get this square. I took the can off and really checked it out. It just doesn't seem to snug in to the header. It's like the space in the header is wider than the midpipe just enough that it causes the extra wiggle room. My guess is the gauge of the metal on the aftermarket cans is super thin to save weight, thus, when putting it into the stock header it doesn't take up as much room. That's just my thought, probably wrong though.
I got the idea to go to the local auto part store and see what I could find. They have a 'muffler fix' kit available for a few bucks. It's a little kit that contains a strip of super heat resistant material that you use to patch holes on a muffler. It won't catch fire. The kit also comes with other crap you won't need, I just wanted that heat resistant material. I cut a small strip, just wide enough to go around the bottom of the midpipe with no overlap. I then held that piece in place and worked the midpipe into the header. There was some excess sticking out when I was done so I took a knife and trimmed it off. You can't see it at all. I then bolted the can back up and low and behold, no more wiggle. I just needed something to help take up space down there and this worked perfect. Problem solved.