What watt light are you using? I'm assuming its an HPS. What cfm is your fan? Is the room that the tent is inside also ventilated. What are the ambient temps like in your part of the country right now? First week of flowering is not too late to try some super cropping to control height. Also a good time to consider if you want to top the plant in hopes of encouraging multiple top colas.
You can construct chambers or tents by making a frame out of pvc and covering it with black and white poly (panda film). Use a closet dowel instead of pvc for the piece that your hood will hang from. Even with sch 80 pvc the weight and heat will make pvc bend over time. Just make the cube (rectangular cube) and then use gorilla tape and panda film to cover the structure like wrapping a giant present. Then get some stick on zippers fro the hardware store to make one side a hugh door.
http://www.youtube.com/watch?v=ZsSofS6lcjM
Cut some holes for your ducting and thats it.
DO NOT use plant growth regulators!
https://www.youtube.com/watch?v=_J8q2vDGz5k
Send me a pm if you want more help, good luck.