Different crock pot types on Shabbat

Question

If I understand correctly, Rav Abadi holds that if one wants to add water to the pot, or to return it once it is removed, than aluminum foil balls are placed at the base of the pot elevating it somewhat above its holder. Covering or removing the controls is meaningless. Does this hold true for all types of crock pots, even those that merely rest on a flat base? Or is it just for those inserted into a round metal pot-shaped holder? Thank you for your time and effort.