I'm not too familiar with the specifics of CO2 in a grow room but I know that CO2 is heavier than oxygen so you would want your source above the plants so the gas can 'fall' onto them, if that makes sense. If you have the brick in a styrofoam container try drilling holes in the bottom of it and placing the whole unit on a shelf or something. Dry ice 'melts' into a gas so you don't have to worry about any mess.