I've been brainstorming about this. I seem to remember my dad saying when he was a kid they wrapped a hot brick and took it to bed with them.
As it turns out, water has much more "thermal mass" than bricks, concrete, stones, etc. http://www.yourhome.gov.au/technical/fs49.html
Problem is, you can't heat water over 212F. Then I thought, hey - fill a pressure cooker 2/3 full, heat it on a stove or fire and when is starts rocking, wrap it in a blanket (whose purpose is to slow the heat transfer and keep yourself from getting burned. Set it in the corner and go to bed.
Well that's my brainstorm or it may be a brainfart. Any thoughts?