In these pictures you can see how I cut out the radii in these parts. I find step drills and hole saws take too long and heat up the aluminum. I have to stop a dozen times or more to keep the aluminum from getting too hot. And even then I can't be too precise.
So I cut out the interior areas with the band saw, cutting multiple lines and then coming back at an angle to cut them closer to the line. When I get real close I can carefully nibble away with the band saw.
It then just takes a round rasp file to get the radius right, then some finish file work and I'm done.
Especially in thick materials I find this method faster and more precise than a step drill or hole saw
