Crock pots with adjustable temperatures:
Foil is to be placed between the outer cooker and the inner pot. However, one is to beware not to place more than a single sheet of foil, as this can constitute a problem of Hatmana/insulating. [One should also cover the knob of the crock pot.]
Must the entire area of the crock pot be covered?
Only the areas that are under the pots of food need to be covered. [Thus, by a crock pot that the walls of the pot touch the entire inside, all of the insides must be covered, although as already explained in the previous questions, if there are small holes, this does not pose a problem.]
Must the knob of the crockpot be covered?
From the letter of the law, it suffices to cover the flame alone. Nonetheless, there are Poskim who rule that the knobs should also be covered. According to all it does not suffice to only cover the knobs and not the flame.
 Shabbos Kehalacha pages 300-302
 Shabbos Kehalacha Vol. 1 pages 305-306
 Shabbos Kehalacha Vol. 1 pages 306
 So rules Shemiras Shabbos Kehilchasa, Igros Moshe 1:93 and other Poskim, based on the fact that the covering is only needed as a recognition.
The reason: As the Sages did not suspect that one would add fuel to the fire and only suspected that one would stoke the currently existing flame. [Igros Moshe ibid]
 Igros Moshe ibid, and Sheivet Haleivi, in order so the area from where one raises the flame also have a recognition.