You would have better success I think if you buy a small paintball tank of CO2, or go to a welding supply store and buy canisters of CO2. No waste, and much easier to control the amount of CO2. Please, check out the stickies, they are very informative and well written on how to make a chamber.

And no, dont just freeze rats or mice, the only humane size for freezing are pinkies. They are not susceptable to CO2, and freezing them is considered the humane way of putting them down.