Did a little work for a guy awhile back and to pay me back he agreed to weld up some angle iron molds. Just got the first two last week, one 6" the other 5 1/4" for some reason. Wouldn't call it welding, more like bubble gumming them together, however they worked like a charm. 1 1/2" angle, welded three abreast, with the same angle closing off the ends with some draft to aid releasing the ingot. The 6" ingots weighed 2 lbs, the 5 1/4" 1 5/8 lbs. One real advantage aside from ease of stacking is that they seem to cool much faster than the muffin tins allowing for a faster pace in emptying a large batch of alloy. They have more air under and around the mold and the elongated shape alloys them to setup much faster.
Now to get some 2" channel iron for bulk storage ingots. My muffin tins are starting to go to pieces, time to replace them.