Two-part silicone mould. Have the piece on an angle; clay it up that way. Standing on its pour spout, basically. Build your dam, pour your silicone, when set take the piece out, Vaseline everything, de-clay the piece and pop it back into the mould, wipe down excess Vaseline, pour the inside half of the mould with only the pour spout clay left in place. Because you've set the piece on an angle, when you pour castings air will rise up the pour spout with few bubble traps.