One alternative to carefully controlling the temperature of a water bath on a stove would be a sous vide-ish setup. Sous vide is a cooking method that uses a temp-controlled water bath and vacuum packed food to get precise cooking. The sous vide devices will hold a water bath between 100°F and around 200°F. The jar of everclear+ABV can safely sit in this water bath at whatever temperature you desire. A couple options are the Anova and the Dorkfood, with the Dorkfood working with your slow cooker or any other simple heat source that plugs into it + water bath. I own a Dorkfood and have used it for cooking and beer making successfully.