Open faced mold or closed and intricate several part molds?  Both could work but would be very different in watching the results to perfect the proportions and process. I never tried it but here is the route I would give a chance to:  First use Silicone RTV molds that are flexible and give multiple uses.  Let it cure well before casting and give an airing and break between casts as it may be collecting gas off from the media and weakening or slightly distorting if not allowed to go back to it's original stages.  Then use the powdered graphite mixed with either a clear  epoxy or polyester resin and appropriate catalysts but also mix in a small amount of thinner to the resin.  Acetone works well and the best I know of.  Make the resin as thin as possible as it is going to be a clear base binder and nothing else.   It should be thin enough and minimally used so the graphite will pour like a cream.  If you want to also centrifugal or vacuum assist cast it that would also aid in eliminating any bubbles or inclusions it but may not be necessary.   bpfink