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