I came across this while searching the forums.
viewtopic.php?f=45&t=15300&start=45#p249249Drill 2 holes in the underside of the cover

Seems to be the best fix I can see so far. Ensures that they will not come off.
I just put my new one on (missing this black clip)
And so I am using blutack inside as a holder instead. Plus the 2 zip ties.
As I was testing the new fix and using the headlight washers a few times, the RIGHT hand side cover fell off my car
So I have also drilled 2 holes in the underside of the cover and zip tied that one too (that side still has the black plastic cap)
Seeing how easily the other side came off my car, I suggest at least anyone who has taken their headlights off the car to do the fix before you lose it ($50 and 2 weeks postage for replacement) in case it's been loosened. And I think everyone should do it anyway because it takes 5 mins per side.