A Tub-in-Tub incubator (T.I.T.) is two tubs one placed in another. The bottom tub or the "outer" tub is filled with water, enough so when the top tub or "inner" tub is placed in the water level rises to the bottom tubs rim.
The heating source is normally an aquariam water heater prefferably one that is 150 watt. A dsired is 75-85 degress F.
Rubbermaid (TM) tubs are desired because they are made in rubber and lack the plastic fumes released that can toxic to mycelium.