Baby cookie cutters will delight you with a variety of suitable sizes and price. However, pretty quickly you encounter the first unpleasant discoveries. For example, note that the relief inside the mold is almost never the same detailed and elaborate as the outside. Interesting fine detail and texture inside looks highly stylized.
Buy a variety of goods, pay special attention to their packaging, think about how it talks to adapt to create soap unusual shape and how comfortable will the resulting soap to use.
Then notice that the soap after curing is in no hurry to drop you in the palm of your hand. This is because the plastic is too hard. To remove soap from molds like this, it is necessary to first freeze and then dip in hot water, or to bend each wall of the form and then flip over and press down. Or at least to pick up the edge with a fork or knife blade.