Simple Grain Mill
Farley Anderson demonstrates a variation on a mortar and pestle grain mill, build using 3 metal pipes wired or taped together and a large metal can. He notes that the pipes allow for airflow that reduces scattering each time you hit the grain.
In Africa, where using a mortar and pestle is a traditional way to pound grain they mitigate this problem by using a tall mortar.
I would wonder how quickly Anderson’s design would allow you to mill the grain – or how finely – since the actual striking area of the pipes would be small.