It's craft foam board. It's tricky though because you need to lay out a pattern first to have it fit right around a waist or chest. So I use paper for the measurements and trace the pattern onto the foam board. You don't want to waste material with trial and error, do that with the paper.
The figures contained in the Custom Showcase are not for sale by Figure Realm, and the creators of the customs are no way affiliated with Figure Realm. The Custom Showcase is purely for entertainment purposes and only provides a means for the creators to show off their work and trade ideas. Figure Realm does not make or sell customs.