Barry Cunliffe's book about Pytheas and the ancient tin trade has an illustration of some copper ingots excavated in Britain in the early 20th Century. The locals tied the ingots two at a time --one on either side-- to the back of mules using the 'legs' of the oxhide ingots at convenient tie-off points. Apparently it worked beautifully. No one told the locals to do it. It was intuitive.

Tin, meanwhile, was often cast in bun-shapes with concave bottoms and convex tops. Based on wrecks, it looks like that was to aid in stacking in the hold of a ship.

EDIT: Added a link to the book and spotted I said concave and concave instead of concave and convex.