Another option is putting the mason jar in an ice bath. I've used this method with both dry ice and crushed "wet" ice, mostly because I didn't have a thermos handy. Dry ice kept the butane extremely cold- actually too cold, since yield was disappointingly low. Crushed ice seemed to be the best option: I would fill a small cooler with crushed ice cubes, then partially submerge the jar. Pull it out in 30min-1hr and you're good to go.