Cardboard is great to use, the lights do not create a fire hazard, CFLs do not get hot enough to catch even paper on fire. Hotglue works really well to glue cardboard together, and it holds strong, if you need it. I made a hood with a vanity light strip with 5 sockets, out of some cardboard, hotglue, and duct tape, plus the vanity strip I wired myself. I lined the inside with mylar [shiny reflective wrapping paper, on the silver side], it works extremely well, look at my gallery towards the newest pictures to see it.
I did have 12 cfls running but the temperature was almost 100, so I cut it down to 2 40 watt cfls and 2 26 watt, and the temperature goes between 77-83 degrees, it seems to be enough light for 3 plants, no streching or anything.