Twin Size KD Mattress Foundation/Box Spring. Dimensions: Twin Size. Material: Steel Construction Frame. Twin Size KD Mattress Foundation/Box Spring. Introducing our new KD mattress foundation. This foundation is small enough that it can be shipped non freight. Made from a steel construction frame for durability and designed so no. tools are required for the simple assembly. This is perfect match for all our premier bedding mattresses. Assembly Required. Also available in Twin Long, Full, Queen, King and Cal King.