I had several made up to make 4# bars to feed my Magma Auto cast machine. My welder (thanks Johnny!) suggested cutting channel iron in his chop saw at an angle. The sides of the channel iron already have "draft" for releasing. He welded 1/4" plate a couple of inches extra long on each end... only on the outside. The cuts were at an angle so the end plates had draft too. The extra long tends gave tabs to pick them up with tongs and somewhere to rap the mold on the floor to drop the ingot free. I was making Lyman#2 alloy in 50# batches and could keep my 6 ingot molds in action.
PS I was initially worried that the end plates weren't welded on the inside but it was nevr a problem.