when making "charing" items such as cloth you need no air at all. when heated in the tin the item starts to burn, this is all the crud and rubbish other than carbon, this rubbish turns into a gas due to the heat, this gas normaly catches fire and burns off, the only reason you need an exaust is to let out this gas but a very small crack or slit would be fine.........
when all the rubbish has burned of remove from the heat, ensure no air/oxygen can get to the "chared" item, if this happens it will carry on burning, just like a piece of bbq charcoal, no matter what it is made from.....
sand was just an example you could use normal mud just as easy, as long as it it not a peaty soil which will burn you would be fine.......... i just said about the sand as you wanted a natural way of doing the job, you also said you did not want to process any thing, (like cloth) so i thought the clay pot would be out??????? also with the sand/mud idea you dont need to carry any thing at all appart from your flint and steel.....
i hope this helps you understand the process a bit better......
sorry for the rant.....lol.....
chris........................................................................