A different option for a spare that I really like is a wallet key. You can have a key made that's flat and doesn't have a blob on the end that fits in wallet.
I have a factory wallet key that came with my truck and it's definitely come in handy.
With the security system in my Dodge Ram, a key without the built-in remote will set off the alarm every time. They can't program it otherwise. A honking horn is better than having to break a window, though. I had a spare key made that will open the doors and start the truck but it sure wakes up the neighborhood.
With any kind of box under the vehicle, I suggest wrapping the key in Saran Wrap inside the box. Water goes everywhere under a car.
