As I have never worked with this stuff but have worked extrusions I have a couple of thoughts I would like comments on...
Alum extrusions I have worked with tend to be stiff as hell but also tend to be a bit prone to ringing because of it. Building a mill from it I am afraid it might tend to induce this into the part especially if you don't have a very high speed spindle. So trying to look ahead I was thinking of filling the center of the extrusion with something to help add a bit of mass and vibration damping material. I had two thoughts my first was to use a machine non shrink grout that we use when we install big machine tools after the final alignment we grout the rails to insure they don’t move. This stuff is like a fancy portland cement. Its messy and after thinking a bit I thought why not just plug one end with something like hot glue or epoxy about 1/4 -1/2 inch then fill it with a very fine grade sand, vibrate it to pack it in well then hot glue or epoxy the other end to seal it in. My thoughts were you would get the ease of working with the extrusion but the sand would help dampen out the ringing that thin walled material tends to have and a bit of mass would be added to the overall build...
Just a thought …
Mark