Milling the plates would not be a problem. Though for 12 mm bolts you want a bit greater then a 12 mm hole as a pass through. Also if you want to use ali' for the axle bar forging without special equipment (read very expensive last I checked) will not turn out well. Aluminium tends to work better if either cast to shape, cast and milled, or milled. Forging of ali' as done when making ali' profile requires a very good amount of heat and pressure to get a good result (reason the stuff costs so much).

Michael