I had a 92 Caddy that had an aftermarket alarm system in it and I was ALWAYS getting locked out, so I unscrewed the license plate and put a spare trunk key behind the plate. I would just use a dime to unscrew the plate and get it. I did have to open the key ring hole in the key so the plate screw would pass through it.
I locked my keys in my van once, the cops came to open the doors. As he was doing his thing he said you really should get a spare key. I laughed, he looked at me as I reached across to the other seat and picked up a small brown bag. Inside I had 11 keys. Ya, I got a deal. I just had them made.