Having a car with no boot, windows and no door handles (so no locks either), I wear a very unfashonable "bum bag" all weekend, with passport, money, credit cards, car keys etc. in it. When I'm asleep, it's in the sleeping bag with me. Everything else of any value is up the oposite side of the tent to the door, although I suppose they could cut through that end if they wanted. Seems to work for me.