Has anyone used these vents on their T3 before? Do they do a lot to reduce the temperature inside the van? The reason I ask is because we have no way of allowing fresh air in without risking someone unlocking the door and these look like the perfect solution. The only problem is that we have a curtain that hangs across behind the front seats and I'm worried that the 15-20 cms between the curtain and the roof won't be enough to allow the air into the back of the van. Anyone got a set of these that they have had good results from? Any other (cheaper) ideas to allow fresh air in without allowing people access to the locks on the doors?