Have you ruled out using flexible material for your casting mold?
I use silicon molding material to make molds for common sign details. Silicon will not stick to semi rigid pourable plastics which I use for the positive. There are about 30 shore variations in the line I use from very flexible plastic all the way up to sledgehammer proof. If your positve is silicon based then flip your mold to a plastic base - this removes the need to seal your mold prior to use. If you pour silicon to silicon or plastic to plastic you need to use mold release.. or you have just glued your mold together.
One of the nice things about using flexible molds is that you can tweek the mold at any time. I made a custom gun grip mold with a candle wax puddle hole which I engrave initials in prior to pouring.
Drop me a line if you need specific recommendations & suppliers. I am not an expert on casting but I've blown through enough test kits to steer you in the right direction.
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