hey man, just my .02 but.... i stick to the sayin "if you go cheap, you grow cheap as well" i would not use something like cardboard around anything electric when im spraying water, etc.... high times had a article in last months edition (i belive it was 06.2006) it talks about a kid making a "HPS" growbox with a 4ft. dresser and 2 computer fans...
do something like that, you can get a dresser at goodwill or something for like $30-$50 , gut it, glue the fronts on and set up a secured back door on hinges and "POOF" you've got a "stelth box"

but then again, thats just my 2 cents