A big decision when making soap is what mold to use. The mold type can determine the size and shape of your finished soap. Not all molds are created equal, some are single purpose and others multi-purpose. Melt and Pour soap bases versus Cold and Hot Process soap can require different molds.
Melt and pour soap molds can be plastic because when the soap is done cooling it is hard and ready to use. Plastic molds are available in fun shapes to add some extra flare to your soap. Silicon molds can also be used for melt and pour soap bases.
On the other hand cold and hot process soap is still soft and takes weeks to properly cure before use. Therefore if you were to pour cp/hp soap into a plastic mold it would stick to the edges of the mold when you went to release it. The easiest mold type for cp/hp soap is silicone, it easily pulls away from the soap yet is thick enough to hold its shape. Wooden boxes, cardboard boxes and other materials can also be used as a soap mold as long as they are properly lined. If you do not line the box, the soap could leak or get stuck in the mold. Overall, there are endless limits to what can be used as a soap mold as long as they are properly lined.