As I said sometime ago I got a spare key made £5 that is flat and I keep it in my credit cards.Someone posted recently a new key was about £30.
I think I may also next time I see a matching rear light unit on ebay they always seem cheaper for an odd one I will buy it and keep in the boot or even at home.
I dont see any problem in the "treffen method" being in the public domain. If they have managed to get into the car un-noticed in the first place by getting past the alarm , and they have the time, location and tools then isnt it quite obvious the cars either already been heavily comprimised ? then again suppose if theres nothing in the interior worth stealing other than the stereo the contents of the glove box or the cubby boxes they could always try to steal the groceries in the boot after a few hours hassle. They WONT be breaking into the boot chasing a set of keys.