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That's what I do. My 2018 Mazda 3 has a physical key inside a fob with a proximity sensor. You actually can lock the fob inside the car by using the key instead of hitting the lock button and closing the door, but that drains the battery on the fob. So I got one of those pouches on Amazon and I lock my fob in there and clip the physical key to a D-ring. I actually got a second physical key made fairly cheaply (without an extra fob) so it can just stay on my BC and I don't risk forgetting. I do try to rinse the key after diving, before sticking it in the lock; if there are no showers, I'll at least lick the salt off.Put it in an RF blocking bag. You can find them on Amazon to help prevent key relay attacks. Then lock the door with a manual key that you can get made for like $3 at a locksmith. Bolt snap that into a safe pocket.
My truck has the combination lock, so I leave my keys in the center console, lock the truck with the keypad and unlock it with the keypad when I get back. Only problem is my memory isn't what it used to be, and after a day of diving I have stood outside my truck looking really stupid trying to remember the code. (I do have it written down on one of the shocks, and have only had to crawl under the truck once to jog my memory...).
Get a key made that only opens the door manually. Put in BC pocket. Keys with electronics stay in car.