No, it can be fresh play dough. You do not have to let it dry out. Actually it is better fresh because it is easier to remove from around your cast pieces.There is another molding material kind of like play dough its called Plastilina it suppose to be available in hobby stores I have never used it or looked for it. It is a non hardening modeling clay mixed with oil.
You can use banding material like they use to band boxes on pallets with but it is hard to bend into intricate shapes and you have to solder the ends very time consuming. Hope I answered your question if you have any more just post them or pm me I will be happy to help.