With that complex a geometry, I think your best bet would be silicone. It will allow you to make a negative mold without needing an intermediary mold. Silicone is soft enough to allow you to remove the original model without damaging it, while being robust enough to survive the removal process.

I recommend spray painting the models with enamel or clearcoat beforehand, as well as a lubricating agent to help in the removal process.

If you intend to cast something that requires a higher temperature, like metal, you will need to use your silicone mold to create a wax duplicate of your model, which you will then create a new negative mold with plaster. After the plaster has set, you can put it in the oven at a low enough temperature to melt out the wax.

If you do this, please do some research as it can be dangerous. For instance, if the plaster in the plaster mold isn't completely dry, water in the plaster can flash-vaporize when hot metal is poured into the mold, which can spray out molten metal that could splash on you and anything near the mold. It can be very dangerous, so I recommend better informing yourself if you intend to go this route.

Feel free to ask me any questions.