I have made emergency road kits with a wax/cardboard to use as a heater.
This might actually be a pleasant idea to add. In fact I think I will even drop in some vaccum packed coffee into that kit, why not have some extra pleasures to keep you feeling human?
Pretty simple things, at least the basic design... I won't go into all the little things I have tried with them here..
Here in Northern Minnesota (aka frozen hell) it can easily be a downright life or death situation if your car breaks down in the winter.
I use a used can from soups or similar, and cut a piece of cardboard to roll into it, then fill it with wax. This give a hot flame that lasts quite a few hours, with plenty of heat to melt snow, and thaw/cook if needed.
I use these also in camping just to keep them in reasonably new condition.
Sure they may last a long time in normal storage, maybe forever, but they tend to melt a bit here and there in a car, get heard to lite etc, so I use them from time to time. It's also gives me an excuse to make the next generation to see how i can improve here and there and still keep it cheap.
I might try to find a way to mix in some dried coffee grounds, in a test, just to see if they would work without a lot of extra smoke.