Two 25cm x 20cm sheets of white 0.75mm thick Polystyrene.
Plasticard (or Sheet Styrene) is an amazingly versatile material for modeling and converting. It is basically dense polystyrene and is essentially the same type of plastic as you find in many plastic kits.

It works with CA glue as well as polystyrene glue.

Made from High Impact Polystyrene Sheet (HIPS).