Hi,
Since that is actually a fairly common problem, we soon will offer a kit to fix that. Until then, I take a paint stirring stick and break it in half. Then put one half inside at the top of the opening, then the other inside at the bottom of the opening. That should give the ears on the box something to grip. I just did this to 3 of my own outlets.
For more details, check out this article that I wrote:
http://www.mobilehomerepair.com/article12.htm
Mark