put a sign in the window!!!
when me and a mate went to Santa Pod mine was out of action and his was in the garage so we took the courtesy car which was a brand spanking new Nissan Micra absolute ugly heap of shit.
Obviously couldnt just drive in like that so stuck a sign in the window said sumthin like "its a fookin courtesy car ok" then had a massive

printed out underneath
then of course the obligatory "passionford.com".
didnt get any abuse, most laughed and the parkin attendant dude thought they werent very courteous