The thing that I do is to nestle the probe into the substrate but not cover it completely. I use a layer of newsprint paper under my aspen (for ease of cleaning as well as insulation) and the aspen is about fingernail deep, so I know that there isn't much deviation in temps on the floor and on the substrate surface. Also, it's best to place the probe under the hide so a) you have a reading where the snake spends most of its time, and b) you have the hide to hold down the cord and keep the probe mostly in place.