There's lots of other boxes you could have used, some have the clamp built in others have a break away tab that lets you push the wire in but will not let you pull it out.
What is this box for? It does not look like a ceiling fan box if that's what your thinking.
Simplist way to fix that gap is with one of these. http://www.bing.com/images/search?q...ure&qpvt=medalian+for+light+fixture&FORM=IGRE
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