Impossible to get them to stay in place. Even if you do get them to stay in place you have to worry about your snake brushing the substrate away so that there is no insulation and the area gets too hot. If you clear the substrate in that area the snake can poop in it and there is nothing to absorb the poop/urate. Clearing the substrate also could lead to the snake inadvertently brushing substrate back over it, lowering the temperature.
This is the only thing I do not like about snakes. I use a temp gun to make sure the hot sides are the proper temperatures several times a day because the probe is always getting banged around or substrate is getting pushed over the undertank heater.
Nightmare