For the ultimate security, I recommend following these simple Australian steps;
1. print a hard copy of your priv keys onto paper
2. laminate the paper in order to water proof it (reason revealed in step 7)
3. proceed to the nearest river or coastal inlet
4. disperse chum (chum is meat offal and blood)
5. watch for a suitably sized crocodile or shark as they approach the chum
6. roll up priv key paper and clench between your teeth
7. launch yourself at the target creature, shark or crocodile, when it reaches a position you are confident in tackling
8. wrestle shark and/or crocodile (yes, in the tropics you may have both at the same time)
9. upon achieving a suitable clench, remove priv key from your mouth and with some vigour, ram it up the beasts butt hole
10. disengage from the target whilst avoiding becoming lunch.
Now your priv keys are in the ultimate biological safe, you can rest assured that it is unlikely that anyone else will steel them (unless they live nearby, have read the article and like inspecting croc or shark arse holes).
Lol thats stupid, first I thought you were writing something interesting, but i read it through and it sounds silly.
Actually if you already laminated the private key, you can put it into a capsule, and drop it into a lake, but make sure it's anchored to the ground to not get carried away with the current.
Then come back years later and hope that it will be still there. Or you can bury it in the forest.