As mentioned you can cast almost anything in silicone molds since nothing much sticks to silicone.
You don't mention the size or shape of the item, or how lightweight the casting material should be though, and those things could matter.
For example, are you thinking of a silicone "push mold" like most you'd buy, or are you thinking of making a 3-D mold by pouring liquid silicone mold-making material around it, or using 2-part silicone putty to make a push mold (or even a 3D mold)?
For lighter-weight large items, you could use something like hypertufa or Celluclay or papier mache, etc.
For smaller ones you could use paper-based air-dry clays, or a polymer clay like Ultralight or Pluffy, or make the items more like shells than fully solid, or use a lightweight permanent armature inside polymer clay, for example, like tightly-wadded aluminum foil, or expanded polystyrene foam ("Styrofoam") or various other solid or hollow lightweight items/materials depending on the size and shape of your final item.