Add Co2 into my grow room.

I have a 4ftx4ftx6ft grow tent with a 600watt hps light. The light is air cooled and I'm running ducting from one side of the light to the outside of the tent, with the fan pulling the hot air out. In doing this I need to open a flap on the bottom of my tent to let proper air circulation into my room. A friend had told me that buying a Co2 setup would not work because my room is not 100% sealed. I was thinking about making a home made co2 set up. I need some suggestions.

the room needs to be sealed otherwise you will be pulling the co2 out of the box and into the surrounding living quarters. That is not a healthy thing to do for people or animals. Plus with only a 600W lamp in that area I don't see a whole lot of benefit to co2. the more light the plant gets, the more co2 it will use. Just my 2¢

You can make a simple homemade C02 emitter by taking a soda bottle (like a 1 liter soda bottle) and get a an airlock used in home brewing like this: http://store.homebrewheaven.com/airlock-classic-s-shape-p109.aspx (local brew shops carry them for $1-2 or just order one off e-bay or amazon or whatever) then make a hole in the lid and mount the airlock into it airtight with epoxy, hot glue, whatever. Then you take a tablespoon of yeast and a tablespoon of sugar, then add like a an inch of water, shake it up screw the lid on and let it go. It will smell like bread when it's emitting C02 and last anywhere from about 1-2 weeks per yeast load. =)