I build one using an old propane tank, a purpose made refractory cement, and Kaowool. You can buy crucibles for this use on Amazon, or spend some time on a metal casting forum to find the best deals.
Note that steel will dissolve in melted aluminum. Weirdest thing but a cast iron pot won't last long.
Note:
Pop cans are a terrible source of aluminum - the alloy is wrong, and since it's so thin, you oxideize so much of the surface you loose alot of metal. You also need to avoid wrought aluminum - 6061 stock, swaure bars, etc. These alloys don't cast well.
The best source for aluminum to cast, is formerly cast aluminum.
Go to a junkyard - intake manifolds, aluminum wheels, etc are great sources for raw metal.