I miss understood the question! I thought you were trying to emulate the spokes of the wheel not the center hole!
By all means use a broach.
I have done something similar to this on a bench top drill press. You can actually mill a D shaped slug from a round and then press it thru a slug of aluminum. For this particular op you might need a small arbor press (HF sells these).
I would drill a hole as Mcgyver says but make it very shallow and then you have a starting point for your punch. This is how we do it on an ironworker. You’re just going to have to try it on a more Labor-intensive piece. But then that’s the beauty of DIY projects!
