I'm looking for a way of tamperproofing an M4 machine screw. Under normal circumstances I'd just recess it into a counterbore and stick a "warranty void" label over the top. However, this is for harsh environment and a label would obviously be unsuitable. I don't want to use a screw with a fancy "tamperproof" head because anybody can simply buy the appropriate driver. What I need is some kind of plastic cap that gets fitted over the screw head and is impossible to remove without destroying. I can't seem to find a suitable product anywhere... Any ideas where I can get hold of such a thing?