Casting Plaster of Paris in a Mold You Make. Adding Plaster Additive for Increased Strength 

Make Your Mold with ComposiMold, a reusable mold making material. You can remelt and reuse it for as many molds as you like. So after you purchase the ComposiMold (advertisement link: amzn.to/3qpz8PG ), you don't need to buy more.
Plaster of Paris makes great casting material because it is low cost.
The Plaster Additive (amzn.to/43f1pXM) reduces the cure time for stronger castings and better surface finish.