Assuming all the other conditions for Chazara have been met, a crockpot that does not have variable temperature settings is permitted to return to. If it does have a high and low setting, the knob should be covered. In any event, it is praiseworthy to line the base with aluminum foil essentially creating a flexible and curved Blech.
However, there are Poskim who are concerned regarding a different issue, Hatmana. Since the walls of the pot insert are surrounded by hot insulating walls, they consider this included in the Gezeira of insulating a pot with material that increases its heat. This can be avoided by placing three small stones inside the base, insuring that the insert does not come in direct contact with the insulating outer surface. There are also numerous Poskim who do not consider this necessary.