A few ideas for extra security:

* Split the recovery key in two, store each half with a different friend. (If you're feeling fancy, XOR the halves and store that with a third friend, then any two out of three will work.)

* Sneak the key into something you know friends/family won't throw away while you're still alive, like stuck to the back of a sentimental photo in a frame.

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That said, I think I'm wandering from the original "accumulating dusty old drives in a box" scenario, which has a simpler solution: Keep a growing old_drives_keys.txt file on your current (encrypted) main device.