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my best guess is that aluminum is reactive

That's what I guessed, too. I was actually thinking of cooking it in a covered glass casserole dish, since that would be a better fit for my chuck roast. (I don't want to need a gallon of liquid for a small roast floating in the middle of a half-tray.) I have one of those white Corning Ware 8" by 10" oval by 4" deep dishes, with a clear glass cover.

Come to think of it, all of my disposable half-trays are also aluminum.

What do you think about just buying Campbell's Beef Broth in cans, which is already 2:1? Seems like a very cheap and easy shortcut. Is homemade going to be worth the effort?