I'm surprised you're not building the plugs with a cnc router. Seems like that maybe your next project! With one you could, of course, mill the necessary radii & tapers for easy release from the mold. You would still have to do the final finish step for any voids in the wood (unless you went to a machinable plastic).
Will the plugs be for production, or do you make a mold of them and use them as a 'master'?
They look very uniform in color, I'm assuming you completely sealed them with something?
Looking forward to the casting process, would also like to know about the sand and furnace!