Knowing the heat you guys get over in Adelaide you won't be able to block the grill much for the hottest days. Temperature wise. Your likely to encounter the worst case scenario for a ecomodder
Easy to remove is the way to go. You could have a slight block for the full heat of summer. Have an extra section you can slot in for cooler days and night time or short trips. You could come up with a flap that closes itself with air pressure but a spring holds it open at low speed.