I have a 5' long, 2" diameter piece of aluminum round bar. I want to make it at least 30% lighter, while preserving as much rigidity as possible. (The force I'm most concerned about is torsion, so if this is too broad then please substitute torsional rigidity.) While I want the rod to be at least 30% lighter, I'm ultimately after find the best ratio of strength to weight. I have an incredible machine shop that allows me to drill as many holes as desired in any diameter down to 1/16". What pattern of holes should I drill?