What is Reddit's opinion of Hide A Key Under Car Magnet Key Case Large Black?
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https://www.amazon.com/Hide-Under-Magnet-Large-Black/dp/B003P4IH9O

Go by any autozone/car shop and they'll have these. Stash it up under the bumper or wheel well. Tell your buddy where to find it. Should be fine for a few hours.

Alternatively go to airport lost and found and say you found the key and have your friend know what identifying feature is on the key chain. "it's a subaru key with a blucifer minature" etc.

For a none-crazy answer, you can buy a magnetized little key-box and put a spare key in it and mount it under your wheel-well where only you know where it is. Then it's a simple as reaching deep under your car to retrieve it.

They're like 6 and a half bucks on amazon.

I recently got back from a trip around the midwest, Ontario, Quebec, about a month ago. Things that will maybe benefit you...

Use truck stops to your advantage when you see one while driving. Many have showers that are fairly clean and might just cost a few bucks to take a shower which is nice. If you stay in campsites though most decent ones will have showers anyways.
Maybe get one of these just in case you need a spare key. I locked my keys in my car in a small town in Michigan at night and it took me hours to be able to find a locksmith that ended up being 30 minutes away. I wish I would've had one.
If you sleep in your car, make sure you're in a good place for i. Some businesses don't like it and rarely a cop will hassle you but they won't really do much if you just explain your situation. Just make sure your also in a good part of town if you're in a bigger city and use your common sense.
Don't always take main interstates. Take some backroads and see some cooler parts of the country even if it takes a little bit more gas.
You will run into problems along the way. You might get lost in a huge different city or have your car break down or whatever. Random bullshit will happen that you'll have to deal with along this trip but remember that most people are good people that willing to help you, so don't be afraid to go meet strangers.