Welcome! And nice idea. Surely much safer than heating river rocks. <img src="/images/graemlins/smile.gif" alt="" /> I like the idea of using coals and punching holes for airflow. It's a good idea for camping and surely many survival situations where humans have been. You can find cans all over the place, and although not usually suitable for drinking, you could make little hanging heaters.

I've never tried this before, but I'm guessing some old hands here have done something similar at least. Would make a nice way to heat something like a can of soup or boil some water. Might not always be feasible in a true survival situation, but I'll give it a shot the next time I'm at a camp fire. Might be a decent way to keep yourself warm on a walk too... sort of your own custom hand heater.