Igolos wats going on. As far as the cardboard goes, I mean It's possible cause I've seen it done before. Using fishing line or something like that isnt a good idea though lol I saw someone mention zip ties, I was going to say the same thing but they beat me to the punch. Just use the power strips, buy you some of those plug in sockets to screw ur 42 watters in a ziptie the power stips to the side of the walls and top. Just make sure you have a sturdy box but that really shouldnt be a problem. AND LASTLY...if you DO decide to use cardboard, put you some type of ventilation in there. 42 watters give off a lil more heat than the normal 23 or 26 watt CFL. And you should be fine, I do want to ask how big is this cardboard box you're talkin about working with?
Someone else mentioned rubbermaids and they work good too, a lil more sturdier. You can go snag 2 big 29 gal or above for about $4-7 dollars a piece and stack em. Get one for the bottom half to sit your plants in and the top half to build your lights into but you'd have to follow the same principal with the power strips....OR.....You can go buy a 2-4 light fixture depending on the size of ur rubbermaid or cardboard box. It's all up to you man, use ur imagination sir.
Here's a couple pics of my set up
