Talc us an excellent filler providing much bulk and sanding very easily. A cheap source is generic baby powder. Just check the ingredients are talc, not corn starch.
2-part epoxy resin is available at hardware and craft stores. An example brand is EasyCast. You mix 1 part resin to 1 part hardener and pour into a mold-either silicone or store-bought resin molds....
Polyester Resin is used for moulding such as paperweights (clear version) and can be used along with fibre glass to make glass reinforced plastic (GRP). GRP is used for motorbike top boxes and some...