I have an idea for those worried about it ....
Rather than making holes In The metal to fix a cover instead of the rubber bungs (my first idea)
You can search out something like this that fits in the hole ....
Allthough it is by no means a way to stop headlight theft , They won't be expecting to see a nut and there's a good chance they'll simply abort
Rawlnut and Rubber Nuts,Top Hat Type, Metric : Fastfix Direct : Tools Fasteners and Fixings