Mr Spruce, if you don't mind leaving your back door open, you can use a silicone hose:
http://www.autosiliconehoses.com/silicone-hose-shop/air-ductings/blue-silicone-2-ply-air-ducting.html
Works very well:
Regards
Ben
Hi Ben
Thanks for the photos. This is always an option, but we also have to consider that if anything is pinched from an unlocked van, insurance doesn't cover any of it. Open doors are a temptation in some of the areas we work in not that we have had any issues so far.
When I was in the motor trade, there we reports of the rear doors being ripped off a couple of vans by gusts of wind. The manufacturers, Fiat, were 'forced' to reinforce the structure around the hinges to stop this happening and then redesign that rear section in later models so it wouldn't happen again.
I guess every time the wind rattles an open door when I'm putting stuff away I have visions of my doors blowing away. Many years ago we were following a van that my Dad had bought back from the PWD auctions in Africa. The bonnet came loose and flew up into the air like a bird. Even today, our friend, who was driving the van back in 1967, has the incident indelibly edged on his brain.
This isn't for me, it for a fellow window cleaner friend of mine. Like me he sometimes has his dog in the van, so needs to have a proper vent IMHO.