I did have the same issue (was like that when I got the car), I managed to very skillfully snap it into place and its stayed put ever since. Have another look and see if you can fasten it place as it might not be attaching correctly hence why its coming loose all the time. I can't recall if the little bracket ears went over the outside edges of the box...
But as your missing it what is your plan find another from the wreckers or buy from VW?
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