Hello,
I think you will be better off to use a larger crucible. You might be able to do what you suggest if the casting does not need to meet any special requirements like water tight etc., but I would think you might want to create some grooves or other feature for the poured metal to grab onto and hold it secure. I assume you are melting aluminum to go on top of the alu sheet? Will a crucible large enough fit in your furnace?