you have a point with hypothermia, however the path to it is actually quite painful.
Gasing seems to be the most humane way, but seperating the spinal column is also very effective for a quick death with minimal suffering. If you severe the spinal column of anything, they can't feel pain, because that nerve sensation can't get to the brain, and without the brain, the heart stops, the lungs stop, and the animal dies.
Gasing actually does make them go to sleep, which is what I think you were originally aiming for. To do this, get a CO2 tank from a paintball store, (i would go with a 20oz), you will need a release valve and tubing. Take a container that you can seal tightly, air tight. Drill a hole at the base of the container an silicone the hose from the CO2 tank to the "death chamber." Next drill a hole in the top, and attach another hose to the container. the other end of this hose goes in a tub/vat of water. This is the pressure release, and will allow the oxygen to escape, but not let any air back in.CO2 is heavier than O2, hense why you put the CO2 hose at the bottom and the release hose at the top(this also saves on the amount of CO2 you use). The animals die through a somewhat gradual oxygen depervation. They get tired, because of the lack of oxygen to the brain. They fall asleep, and die shortly after. Loss of bowel control is common at death, so line your death chamber accordingly.
I think my harsh way of criticism was out of order, and I am sorry for my flipping out. I tend to do that from time to time.
I think that you will see that this is a much better action.
P.S.
1. The snow is OUTSIDE of my house.
2. If you want to know the pain that I am talking about with freezing to death, open your freezer, and place the back of your hand on the bare metal rack that is in there(if you have one) and dont move it for a few minutes.
I took JM's pic from the gallery, to show you what i was talking about. I hope he doesnt mind.

This is JM's Deathmaster 3000, for visualization