With Jetrangers working in the artic, we used to put car warmers (those little fan powered plug-in heaters) on the floor in the cockpit, that worked well. Although not enviromentally friendly, try carrying a jug or two of car-windshield washer fluid, the winter stuff thats good to minus 40, and pour that over frosted or lightly iced plexiglass windows, works better if its warm. Check with your engineer first though. Also, we always killed the a/c heaters before final so the windows had a chance to cool before landing, otherwise blowing snow would melt on them if they were too warm then freeze and make a hell of a mess. Good luck!