Pouring flat sheets is much tougher than it seems. You can "puddle" the brass on sand, very fine casting sand with a little wyoming bentonite and appropriate moisture level, will give a relatively smooth surface. But, you will need to preheat the sand form and whenever I have seen it done in art foundry they make a long trough to pour the brass into and gate it over into the sheet area. The result will be thick and voids and flaws are hard to avoid. The old timey method is to then hammer the sheet out to the thickness you with frequent annealing. Having watched the process several times may have influenced my tendency to iron mounted rifles.
