I’ve been experimenting with “modifiers” for my 99 PAO (which is actually rather a POL) for a good while now.
With use, my system evolved to the following “themes” to add one digit :
9 (frozen, cold but dry): locus full of snow/ice, objects are frozen, people are bundled up for the cold with furs, heavy winter coats, thick mittens making it hard for them to handle certain objects, etc
8 (Vinéa/sci-fi): locus futuristic, objects are only holograms or any futuristic version of them (think hoverboard instead of skateboard), people wearing cosmonaut/vinean suit (or any costume from a civilisation from the future/highly technologic, or cyberpunk/solarpunk)
7 (volcano/fire): locus on fire / with fires or lava, smelling with sulphur, objects red hot or at least too hot to be handled, or fossilized and crumbling, people: firefighter or hazmat suit
6 (mountains/Alps): locus mountain like (cragged peaks in the background, névé/pockets of remaining snow, ups and downs, cows or goats pasturing, etc.), objects carved out of wood, with tons of embellishments, people like armaillis (big beard, loud voice, offering cheese, swiss costume, playing alphorn, yoldeling…)
5 (desert, hot and dry): locus hot and dusty, objects are hot and dirty (uncomfortable but not so hot that you can’t handle them), people wear Bedouin-like clothes
4 (jungle, hot and humid): lots of plants/vine/creepers/flowers everywhere, objects covered with moss or insects, people: explorer’s white hat and machete
3 (sea): locus becomes beach like or is underwater, you hear the waves, objects are slimy covered in seaweeds or encrusted with salt, people have some sort of bathing costume or diving equipment suit, mask…
2 (Ireland, green and rainy, cold and wet): locus is cold, rainy and muddy, objects are wet/muddy, people with umbrella, raincoat, rubber boots
1 (Altiplano): locus is super windy, objects are used or broken up, people wearing poncho, chullo
0 (dark): locus is dark (nighttime, or cavelike/troglodyte, mines), people are wearing spelunking headlamp or other equipment, objects are sparkling with St Elmo’s fire and providing light to the locus
Benefits: since you only need 10 themes, it’s quick to choose them. Since they’re themes, there can be variations and it helps counteract a bit the “sameness” problem. The same object with the same modifier can be different depending where it is placed, who uses it, what is the number that you remember (page number to find a song, or atomic number…), etc.
Inconvenient: it feels “bulky”, a bit awkward sometimes and a bit slow. Speed is not the main criteria for me because I don’t do memory sports (I’m more in the “memorisation for a life time” perspective), but that doesn’t mean speed isn’t important (especially in decoding).
Because of this and because I use my P(A)O a lot (and often need 3 digits numbers), I’m in the process of building (and training) a 3 digits one. It takes a while but I figure it’s worth it.