BruceK, I'll bet you think your friends call you names like "paranoid," and that they do it because they are against you.
Seriously, I prefer a locking cap too. It is a safe bet that the filler is the same for the Cooper, so maybe some kind MINI owner could measure the opening and post it so you could see if a locking cap available locally would fit. MINI will probably offer one, but at a premium price. I don't recall seeing a lock on the flap on a Cooper, so I wonder if they have a locking cap inside. I doubt it. If gas prices go back up (and you know they will), we can expect to see people using a short piece of garden hose and siphoning gas from someone else's tank. Far worse is some jerk dumping moth crystals or sugar in the tank as a practical joke. Engine damage isn't funny.