The inspiration is definitely Miyazaki, but we were more thinking about the Susuwatari (noiraude in French) from "My Neighbor Totoro". Although I would not be surprised if they made an appearence in Spirited Away :) !
You are right about the difficulty, I didn't have sufficient time for proper testing...
EDIT : After proper research (ahem), you are right They do have a major part in Spirited Away. I don't know how I forgot about it. Anyway, nice catch !