How about this concept, a bit more expensive but lower maintenance.
Take a mini freezer (such as
this one) and attach a Vent with a muffin fan on it. Inside you normally would have just bottles of water, but dry ice could be used for CO2. You would probably want to open and close the vent to the freezer so you do not strain the motor(s).
Edit: or you could turn the freezer off when you are venting out, but turn it back on to re-freeze with the vent closed.