Source? If you consider it to be hatmono it goes by rov
To clarify, you are correct that for there not to be הטמנה (according to the שיטה that it's הטמנה despite the top not having a cover besides the lid) there would need to be רוב uncovered. However, that is because a crockpot, with the top exposed, is a normal way to cook, so the typical partial exposure doesn't help.
However, artificially raising the pot by putting foil balls (some use a tuna can or a spoon) should work because the exposure of the top inch or two of the walls of the pot is not typical. כנלע"ד.