For a real cheapo ingot mould one can use the small angle iron used for bed frames. There seems to be an endless supply on the curb on trashday. They can be either welded together upside down to form "w"s, or "v"s can be cut in a 2X4 to suport the angle. Use the cut out v's to block the ends.
Pros and cons, This will make some pretty long ingots which can be cut with an axe when needed. This adds another step. Good thing is they can be stacked vertically which saves space. Imagine all the possibilities, NJMJ