Refitting the side mirror casing

I knew it was a matter of time before I got my first ding. This one was self-inflicted. Backing out of the garage, I lightly clipped the passenger's side mirror. I could immediately tell the casing was loose, so I got out and put it back on (or so I thought). Later in the same day I hear some rattling on the right side and look over to see the casing flopping in the wind. I pull over quickly and realize that it was loose again; I was also surprised it didn't just fly away as there wasn't much holding it on.

I spent about 5 minutes figuring out how to put it back on, and this time it snapped fully into place, with a tight fit around the mirror and the underneath. I compared it to the other side just to make sure it was right. Only slightly worser for the wear, as there is only a small scratch on the casing.