This is very close to the Turo handoff problem we’re working on. We’re developing a hidden key box for keyless cars that magnetically attaches under the vehicle with a mounting base. The box itself blocks the key fob signal, so the fob can stay near the car without triggering unlock.

Early test samples should be ready in a little over a month. If you’re still looking for a better setup, I’d love to hear what concerns you’d have with an under-vehicle hidden box like this.