I carry a large tin coffee can in my car along with a smaller one.
Seriously, a good idea... I did deep woods survival for quite a long while in my life, and the single most difficult thing to overcome is something to boil water in.
I don't put much stock in carrying all kinds of stuff, preferring to have the tools to get what I need from the land... If you don't have tin, you're down to dropping hot rocks in a wooden bowl or something... That tin cup is a godsend...