I use a cable with a built in combination lock that I bought in a camping shop. It's long enough to pass through the sleeves of coats and the steering wheel, useful if we've got the flying jackets but don't want to walk around with them.
I left my rather nice travel bag behind Roger-Cheshire's Morgan's seats unsecured while at TOTH on Saturday and pleased to say it was still there 8 hours later which I was pleasantly surprised about as there were some really dodgy characters roaming around mainly wearing helmets to hide their faces!!