Surf keys have been around for a long time now. They are simple to have cut, inexpensive and supa handy.
Surfers have them cut so they can pop them in a small velcro pocket of their boardies. They can lock all other keys, wallet and belongings in the car for safe keeping and simply enjoy the surf without any worries.
I use one because I often take a friend camping with me. Giving them a surf key means they can get in and out of the car, day or night, without seeking me out. Fabulous option for the kids. If they lose it swimming in a creek, running through the bush or worse.. drop it down the composting loo.. Noooooooo!!!! It is not a problem. Just leave it there! Simply have another surf key cut when you return to civilisation.