We've been using a crew mans harness from Lifesaving Systems Corp for years for crew ops in the back including photography. it works really well. It has a double safety lock with fully adjustable length strap and once this is anchored in the aircraft using the quick release (with safety), they are not going anywhere - hopefully
They are available from Life Saving Systems. As it happens, I have a couple of them which i no longer need - if you're interested PM me or of course LSC will be happy to sell you a new one. They provide pretty good service.