We have tested several insulated containers and had the best results with the 8" deep Cambro Insulated Food Carrier which is used by caterers to keep food on stainless steel trays hot or cold when transporting it to other locations.
Before that we used various picnic coolers and they work well also but need some extra insulation on the top (a small blanket). A pan with a tall rim can be floated on the water and a pan with a short rim can be supported by 4 water glasses (6") - just keep the water level 1 mm higher than the top of the glasses.
The only thing that needs to be done to make the Cambro into an incubator, is to drill two holes (downward) into the side, one for the air tubing and a slightly larger one for the electrical cord of the heater (which can be spliced together again on the outside). The larger hole is also big enough for the thermocouple (which came with our heater) and it has a little extra room to let the air exit which is pumped into the water with the air pump (connected to air tubing and a 4" air stone).
The lower black line is 35 mm below the upper one and marks the beginning water level. Fill with water to this line and then put the empty tray into the incubator. Then slowly pour a little more water into the depression on the opposite side until the tray floats up about 2 - 3 mm. Take the tray out slowly and when the water is calm, put a piece of tape at the water line to mark the final water level (draw another line there - the red line - when the whole area has dried off). This will be the water level from now on. After two batches drain the incubator and let it dry out. Keep the air stone wet because if it dries out, minerals will deposit in the small holes and it will plug up and not let the air through.
That's it -- you now have a proven and reliable tempeh incubator. Put the lid on it and let it run for 24 hrs to make sure the water temperature stays at 89˚F. It is always good to have another thermometer to double check whether or not you are getting a true reading (Our digital heater heats 3˚F higher than indicated).
The air coming into the air stone gets warmed by the water and humidified; it also helps to circulate the water so that the temperature is even.