So, at risk of prolonging the Muppet show. how would other people do this? In wood, I would probably throw a quick CAD model together in something like Fusion360, produce an STL model from it, then throw it at Vectric Cut3D to produce toolpaths. Rough with flat-ended cutters, then finish with whatever largest radius ball-end milling cutter I could find in the box. F360 could probably produce the toolpaths as well but I haven't played with the CAM stuff in it yet. Depending on finish required, probably end up with sandpaper wrapped round a dowel...

In aluminium, depends on what it's going to be used for. Clearance? Finish not so important so might do similar to above. Bearing surface? Try to rig up something on the vertical mill and use a large-radius boring bar of some kind. Not CAD but geometrically better? Good finish needed for cosmetic reasons but accuracy not so important? Maybe use "wood" technique and then find a way to file/polish out the ridges?