Iv'e seen the same problem and solved it easily enough with a large metal coffee can. I put about 2" of lake water in the can and then put the can over top of my heater until it is at about body temperature. Then I place the propane bottle in the can and the sluggish heat output picks up as if it were summer time. This method works for me because I use a single burner propane camp stove where the bottle stays in a vertical position. This may not work for every setup but you should still be able to warm up a frozen bottle by this method.
