Thank you for your answers.
I am not at my normal place, but at a location where I don't have my Rotex, and have to work outdoor, pieces being held by SuperJaws xxl (so container below is not easy, with a little breeze carrying away the fine dust).
In the end I managed to create the fine dust by carving with the Festool Vecturo multitool (I understand it to be actually made by the Fein company).