Do to some recent and even non-recent articles about the freezing method to euthanize reptiles I've stopped that practice.
The last thing I want to do is cause them any pain.
I now use CO2(dry ice).
I purchase a small chunk of it. It's usually wrapped in paper so I place that in a small cooler and place the snake inside after a cool down in the fridge.
I place the lid on and check on the process hours later.
I will try to locate the article, I think it was Reptile Mag.