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* Use a Brew Belt Fermentation Heating Belt and wrap a "hoody" sweater or some other fabric around it to insulate, and a temperature controller.
* Use an aquarium heater (Enheim brand recommended), fill a large plastic cooler with water and keep the water warm with the aquarium heater. Set the souring vessel in the cooler.
* Use a "Seedling Heat Mat" that is used to keep plants warm.
* Use an [http://anovaculinary.com/ Anova sous vide device] - place the vessel in a larger vessel with water, and keep the heat applied to the outside water with the Anova. Works well for maintaining heat for starters too.
* Similar to the Anova, James Spenser of [http://basicbrewing.com/ BasicBrewing.com] video on using an [https://www.youtube.com/watch?v=bquf6EhRe5Q InstantPot® to maintain "yogurt" temperatures for 1 gallon batches].