I asked Mizuno what sort of sheath heater he uses. He uses a 500 W model, such as the Flexible heater "M2 type micro heater" available at this website:
https://www.monotaro.com/g/03032749/ He says they are all pretty much the same, so he goes by price. I would add that it has to withstand high temperatures. I will add this info to the paper. The Monotaro web page Google translates well: Flex Heater "M2 Type Micro Heater" Because the outer diameter is extremely thin, it can be installed in places where mounting is not possible with a sheath heater. The heater is thin and flexible, so it can be easily bent and wound. (Minimum bending radius: 3 times outer diameter of sheath) Finished in a thin tube and has a small heat capacity, so the temperature of the heating element is transmitted immediately. The resistance per unit length is constant. A variety of sheath outer diameter heating elements are available. Sleeves can be attached to both ends according to the customer's usage conditions. The sheath uses a metal capillary SUS316, so it is highly durable even at high temperatures. >
