Cuttlefish wash up on the shores all the time, all thats left when found on the beach is a white oval shape with a soft side and a hard bony side.
What you need:
A few cuttlefish( can be found at the beach or pet stores have them for Parrots!)
Small off cut of hardboard.
Blowtorch or small camping stove.
Step 1: Make the mould.
There are two ways to make the impression in the mould, you can carve away at the soft side with small files and nails or you can use a simple piece of jewellery you have around and push this into the soft side of the mould, so that when its removed it leaves an impression. (see photos)
Flatten the soft side of the bone with some sandpaper wrapped around a bock of wood. Then make the impression about half inch or so down from the top of the flat end of the cuttlefish, then with a file make the pouring sprue from the top of the mould. This is where the molten pewter will be poured in and when finished it gets cut off.
Take a small piece of hardboard that will fit over the back of the cuttlefish, this will become the back of the mould, rub this board lightly over the soft side of the bone so that it sits flat.
In the photo of the cuttlefish with the round shape, Im making a ring with a flat top so this mould has a sprue to pour into and a hole at the base for the metal to flow into another mould for the ring top. (but thats another story!)