Okay, you have a 3 jaw chuck that can grip an ID.
Make up a ring that fits the ID of the part, with a taper, and has reliefs to clear the tabs, and has a cylindrical bore of its own maybe 1" smaller than the workpiece ID.
Sawcut the ring radially in one place so it can expand.
That's the simplest possible fixture you could use.
It may not be possible to machine both ends using it. You can turn it around on the 3-jaw, being careful to not move it relative to the workpiece, and have a chance of doing a decent workpiece in two chuckings.
I was thinking of something more complex, made from say 4-1/2" round x 6" long, where you would chuck the aluminum cylinder in your 3-jaw internally or externally, and machine your taper and maybe a shoulder at its distal end to hold your workpiece. You would also put a conical bore in that end, a blind tapped hole for a central stud, and six or eight radial slots. Insert a conical steel plug and draw it in with the stud to expand the fingers to grip the workpiece. You need to use a taper that's steeper than ~7 degrees per side so it won't be self-locking, or provide means for extraction.
Machine shops used to keep a stack of IBM cards around, even if they didn't have CNC machines, because IBM made the cards tough and hard, and uniformly thick. Modern card stock is probably not as hard or as uniform, but it should still do a decent job of preventing scars on the workpiece. I.e., wrap the fingers with a strip of cardstock before chucking the workpiece. The paper also improves the grip.
Pembroke Pines, FL, USA