I us aspen as substrate but under my hides i use a small towel that i cut out to just a bit larger than the hide. I run the wire for my probe under the substrate and i have a small hole cut in the towel. i tape the wire to the underside of the towel and stick the sensor end of the probe through the hole so that it gets the temperature of the floor of my substrate. it works for me and my BP cant move the probe.