you can try putting chunks of dry ice in a cup of very cold water, it should give you more co2 = very good and should lower your temps a few degrees, not you would need to experiment and figure out the rate of decay of the dry ice, now this many seem annoying but it should not take you more then a few hours and this could increase your yield over the long run. weight a piece of dry ice and drop it in a cup of refrigerated water and see how long it takes to turn 100% to gas. This varies on your location and atmospheric conditions so have fun with your experiment and post your results and ill help you out if you need more help. Oh one more thing, ventilation is a good thing...your plants need fresh air and so do you so if you use the dry ice technique you shouldnt use too much in a confined space because exposure to higher levels of co2 is not good or a long period of time. Good luck and dry ice is cheap!