Step 2: The first cardboard piece
Cut the piece to the length of the CCFL inverter + 4cm (leave some room)
Now put the piece of cardboard around the computer FAN and cut it to length. Again, leave a few cm's over.
Edit: I actually advise to use PVC pipe here now because it's more fire resistant.
If you use PVC pipe, make sure the diameter is just large enough to fit on the FAN.