Funny this got brought up. I have had this issue for years on my hard house. What works for me is a torch connected to a 1 lb propane tank. Works wonders as long its in your truck and not locked in your fish house.
Now the funny.. Saturday night I was pretty wore out and went to bed early, I was staying by myself. Had a couple buddies staying in their wheel houses nearby so I locked my door as to not be anyone’s pranks for the night. All was good till a 25″ hog caused the rattle reel to go off. Caught it, thought all was great! Realized I needed to take a leak and NOW! Lock was FROZEN!! Torch was in the truck.. Panicking, I finally pulled the minnow bucket out of my 5 gal. pale and relieved myself. Never thought I’d get locked in my own fish house..