[olddog]

To my surprise, aluminum pipe plugs are readily available.

McMaster-Carr mcmaster.com has them. A bit pricey at $8.61 each.

Part number 44705K339 1.5" npt.

However aluminum parts, when oxidized, tend to become one part over time. If you try them, I would make certain that the threads are well sealed, with something that keeps them from touching.

I still like the O-ring seal best. I think Manowar said the same thing in another thread.