The locking wheel nuts on my boost alloys have covers (but don't look like the plastic ones in your link). Mine have a special tool for pulling the covers off, I will dig it out tomorrow and post a piccy. Might fit the ones you are looking at. If so you should be able to get one quite easily from a main dealer.
 
I think the tool you mention grabs underneath a recess in the cover - that’s if your covers have nut shaped flats on the end of them? My covers are fully domed with smooth sides so no undercut/recess anywhere for a tool to grab onto.
 
I used similar to cover the bright zinc coated bolt heads when I fitted a new towball. As you say very difficult to remove, although I did get them off with a mole wrench and a cloth to minimise damage.
 

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