However, not all plastic containers are capable of withstanding the temperature of molten basis - 60-70 °C. Some warp and deform. But those that withstand high temperatures for a long time "not live": to retrieve the soap, you will probably have to cut them off.
For the first experiments will fit cups from the yogurt, cottage cheese or sour cream. Small plastic boxes for cookies and blister (solid polymer film) may also have an interesting shape and textured surface.
Useful may even be a plastic bottle: pouring the soap base in cut off the bottom, you will get soap in the shape of a flower, which can safely be called a massage!