What to do when the cover to the potentiometer will not go back in?

So, I think I was messing around too much so I found out that I messed up the green cover where the analog stick is, to where it will not go all the way in.

I'd figure either I could do it properly and replace something, or I could like tape it, so it would be forced to stay in place. Thing is, I don't know where I can buy the replacement part to it. Or the name of it, so I probably cant even find it.

These are what they look like. This is the right stick.I took a picture of both sides. The controller is still semi usable like that, but it drifts (hmm, wonder why.)