Classic.
In my first job I worked on ATM software, and we had a big basement room full of ATMs for test purposes. The part the money is stored in is a modified safe, usually with a traditional dial lock. On the inside of one of them I saw the instructions on how to change the combination. The final instruction was: "Write down the combination and store it safely", then printed in bold: "Not inside the safe!"
awesome !
Hooked from that moment! The series got progressively more ridiculous but what a start!
There is a video from the lock pick lawyer where he receives a padlock in the mail with so much tape that it takes him whole minutes to unpack.
Concrete is nice, other options are piles of soil or brick in front of the door. There probably is a sweet spot where enough concrete slows down an excavator and enough bricks mixed in the soil slows down the shovel. Extra points if there is no place nearby to dump the rubble.
If you just wanted recovery keys that were secure from being used in an ordinary way you can use Shamir to split the key over a couple hard copies stored in safety deposit boxes a couple different locations.