I get that this might be subjective since 略字 themselves originated (mostly) subjectively, but are there any rules for being able to create/use new ones? For example, the 广+マ representing 魔 or 摩 is quite common. This blog post shows 木 + ｷ to represent 機 (which I've seen), but it also shows 艹+ヤ for 薬 (which I hadn't seen).
So can you just make up and use something that would be easily recognizable? Like the other night I was having trouble remembering the stroke order for 華. I thought, "Could I just write 艹+カ for 華・花? And would people understand it?"