Here is what I would propose. Simple, but indecipherable to anyone you didn't tell. Choose one of the categories of little pictures - animals, or food items, or what have you - and simply assign them alphabetically or at random, and then share the list with one or two persons to memorize. No one else will think to consider that a dog was Aoi or a banana Ashiya Douman.

[Plus it would be really funny at trial if Sasazuka was murdered and his phone was just a row of eggplants.]