Finally a serious answer as I've had the same problem! I recently starting using this: 
  
 
  
I've filled 6 small linen "pot pourri bags" with the stuff and placed 2 on the boot, 2 on the soft top and 2 on the bonnet, similar to these but without the flowery stuff: 
  
 
  
  
It does the trick for me and removing 6 small bags from the car isn't a hassle, although they do leave a little dust at first so you may want to know them around a bit before placing on your car. 
  
Available from Amazon & Ebay: 
  
http://www.amazon.co...0?ie=UTF8&psc=1 
  
http://www.ebay.co.u...=item1e7719245f