noddy wrote:

I don't see the potential issue with fumes though. You're driving away from anything you create yourself and the exhausts point down towards the floor.

It's the turbulence created by the vehicle that drags the exhaust fumes into the rear. A bit like when you get spray on the back window when driving in the wet.  

I've driven a few times (albeit quite slowly) with the rear tailgate window open and it's fine. We were coaching some of my triathlete friends on how to group ride together properly. For some reason it seemed more fun to have someone in the back with a supersoaker to spray anyone acting like a twonk  
07 Supercharged - Still breaking her in!

Boot soundproofing part 1 - http://www.fullfatrr.com/forum/topic15612.html
Custom exhaust - http://www.fullfatrr.com/forum/topic15296.html
18" wheels for Brembo equipped cars - http://www.fullfatrr.com/forum/topic15052.html

I'm surprised the Top Gear film crew are still alive then! 2010 MY Vogue SE TDV8 3.6 Stornoway Grey- fully loaded

I think playing with the windows and fan to keep the exhaust gasses out will work, you just need to find out how to do it exactly Something sticking out of the car also has an effect on the slipstream while only the top open has a completely different effect. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!