Zip-ties are cheap.
Ecomodder member Vekke has
invented a new type of zip-tie which looks interesting.
Another type is the bolt that came with my Fabia grill block - it was a bent piece of metal with a screw hole and a screw, the metal hooked behind a grill slat and the end was turned 90 Deg with a hole for a bolt to go through.
or
Or you could use the method shown
here which is removable with a screwdriver as long as you can get behind the grill. The parts are available in most DIY shops.