Euthanization is a touchy subject it is suggested to use Chloroform or Halothane for inhalent purposes to put the animal to sleep quietly. Pour some on a couple of cotton balls place them in a jar and then place the reptile in there with it and after awhile things will take progress. After breathing has stopped wait at least 10 minutes to ensure the reptile has passed.
CO2 has been proven to hurt the eyes and respiratory tract even though it is quicker than inhalents.
We have in the past used the freezer method but we have not euthanised many reptiles and this is what I have come up with some research.