76 degree white coconut oil (not fractionated). A standard in the soapmaking industry, coconut oil creates a durable hard bar of soap.
This comes in a jug, so if it the temperature is under 76 F when you receive it, you will have to melt it in the bottle in order to pour it. This can be done by just leaving the bottle in a warm spot such as near a heating vent, or sunny window. Another option is to fill a sink or large pot with hot water and place the bottle in the water.