A slower, steady supply of CO2 will be much more humane and I think is more effective than a rapid dose. No reason to really rush the process. It may take another minute or two, but just go tend to another thing during the process, and with keeping herps and rats there is always something to do.

Also a less complicated way with less components you can just use a small container drill small holes in sides, lower to the bottom, add dry ice and set it in the container with the rats.

A slow dose will allow for a humane method of euthanization. They will basically just go to sleep opposed to choking.