I don't want to go too far off topic, but agreed, I've always read CO2 is not a humane way to euthanize a reptile. As you noted, vet (usually heart stick) or blunt force head trama are the fastest and generally humane ways to go. I recently found this article (link below) which discusses freezing, freezing on it's own was a widely used practice but not generally humane without first using the fridge, you do not ever want to put a reptile directly into the freezer. At least it's another option if you can't do the first two. I've done all three at one point over the years, it's never an easy thing no matter what method.
http://www.sciencedaily.com/releases...cienceDaily%29
another good source of info on this topic:
https://www.avma.org/KB/Policies/Doc...euthanasia.pdf