> Keep them in a small zip lock bag. If you get very cold you can see them and rip the bag open
> with numb fingers. A 35mm canister will not be so easy when it is frozen.

I'd have thought you could open a 35mm canister just by squeezing it, wacking it or otherwise deforming the body. The lid should just pop off. A ziplock bag you either need to carefully grip opposite ends of the mouth, or somehow tear the plastic, either of which would be hard with cold hands.