Angle iron or flat bar will do the trick, when the mill is set for off loading the bar is resting top dead center so as not to interfere with unloading., the gearbox is something I had laying about and the drive shaft is made up from I believe bits and pieces from a tractor PTO.

The drum for the mill is a crop off of an oil line pipe and is heavy wall, the bung to unload came from a metal barrel, the wheels that the drum ride on came from an old cart.



The balls came from a real ball mill like that shown in my second picture, they're about 3 inches in diameter.

Those large mills above were used at a mining operation to mill the ore, they belong to one of my neighbors. I suspect that he has replaced the armor plate tiles inside and will offer them fro sale it they are not already on the market as mining is not his line of business.