In the quest to figure out the most suitable option, NEVER store them online. Everything online has a backend and you have no clue what goes on there.
What can be the best possible way, My favorite method is to using a metal keycard where I marked the phrase buy again it can be stolen 
Someone can steal it only if you let them, I mean... Have you considered having up to 2 or even 3 metal keycards? Split your words across them and store them in different places (not in the same house). But then, what if 1 piece gets stolen and you do not have the complete words anymore? The solution is that you can get double of each piece (during creation) hence if one piece were to be stolen or misplaced, you can have its duplicate in another location.