I use Al soft drink cans to make "lost" mould ingots, bury in dry sand and fill with molten Al. Thin cans become part of the ingot, the bottom half of which also makes useful round stock. I use any A! tubing I can get my hands on, square, round, cigar tubes. Dunked in water to "heat treat" them. The thicker tubes don't always melt into the ingot, but it doesn't really matter. The top of the ingot will have shrinkage voids and gas bubbles.
Regards, Matthew