u/Angel_in_Flip-flops

One of my talents is locking my key in my campervan (or in my car). I then have to call roadside assistance for help or my wife to bring the spare key.
When we travel together. my wife also brings the spare key along, but I sometimes take a trip alone. What do you suggest for bringing the spare key? Is there a way to place one of those key locker thingies with a number lock under the van in a place I can reach it? Or do you have an alternative? I own a 2001 Volkswagen T4.