It exists already, most likely. I've got a bunch of replaceable pilot counter bores that would get it done, though a multi-flute dedicated cutter would be best. Might want to cut a smaller diameter first and then finish at full diameter. If you are clever, an adjustable stop could be incorporated to get them to same depth on all holes. A custom tool would be a couple hundred bucks professionally done. I could do a one off in my basement. Because it is cutting aluminum, the tool doen't need to be extremely hard to work. I've made a couple dozen custom taps and dies over the years for unusual threads. In aluminum for a couple holes I don't even harden them anymore. O-1 or W-1 tool steel will outlast the aluminum and is easy to hone.