Make a rubber mold using your model.
Purchase an unvulcanized rubber molding compound at your local arts and crafts supply store. There is a variety of molding compounds on the market and which you choose is a matter of personal preference, so you may want to experiment with making molds out of different types until you find one that you like the most.
Press your model into a suitably sized mass of the rubber molding compound. Be sure to surround the model entirely to ensure a complete mold for casting jewelry. The compound will be soft and pliable until you cure it, so there is no need to worry about damaging your model.
Subject the rubber mold to heat according to manufacturer's instructions in order to vulcanize, or cure, the rubber. The finished product will similar in consistency to a rubber bouncy ball.
Use a surgeon's knife to dissect the model out of the mold. Slice down the center of the rubber mold, length-wise along the model, creating a mold that opens like a book. Be careful at this stage not to damage your model with the scalpel.
Remove the model, being very careful to keep the mold intact, as any imperfections in your mold will result in a flawed wax casting.
Cut a shallow channel into each face of the mold, leading to the hollow chamber left by the model. The 2 channels should meet when the mold is closed together, creating a tunnel in which you will insert the syringe.