Why not drill a hole in a wood lath that’s a tight fit for the marble, put a saw cut down the centre to split it like a clamp & then put a bolt at right angles to create the clamping force. To cut the marble use a motorised diamond wheel tile saw?
RonA
Plastic U channel (something like conduit), fill with epoxy, stick in a row of marbles so that they are fully immersed, cut with a tile saw or diamond Dremel.
Assuming they don't stick to the epoxy (!) you'll have two halves of moulds you can superglue the next batch into. Should be able to burn them out and burn off the superglue.