Wrap a cup in elastics and dip it in boiling water for a few minutes.
You can choose to treat the object with rubber. This gives a porcelain-like appearance. Rubber is a two component material.
You need 8-10% hardener. For example, use 100 ml rubber, then you need 10 ml hardener. Stir well so it will become one emulsion.
At a craft store you can find more information about this.
There is also an opportunity to fill the form with plaster. After the plaster is hard, remove the styrofoam.
Choose 2 shapes you like and connect to each other. Drill a hole in each object. Place a metal rod for strength. Connect the objects and fill the edges with plaster.
You can leaf this object as it is or also treat it with rubber. If you add rubber, make sure the plaster is completely dry. Otherwise the moisture can not evaporate.